Like a lot of working-class suburban kids, David Koschman wanted to taste Rush Street, Chicago’s fabled adult playground.

He and four friends — all freshly 21 — headed downtown for a Saturday night of bar hopping.

Around 3 a.m., they decided to call it a night.

As they left Bar Chicago on Division, they crossed paths with a group of three men, all about ten years older, and a woman.

They didn’t know it, but the older men had ties to the most powerful people in Chicago. One was Mayor Daley’s 29-year-old nephew, Richard J. “RJ” Vanecko.

Koschman bumped one of the men, and the groups — both later described as “drunk” and “plastered” — clashed.

What followed were words, insults, and curses — and a punch to Koschman’s face that sent the 5-foot, 5-inch suburbanite to the ground and ultimately cost him his life.

The Cook County Medical Examiner called the fatal brain injury that killed Koschman 12 days later a homicide.

No one was ever charged.

Police and prosecutors concluded Koschman was the aggressor and that whoever hit him acted in self defense. They have never said who punched Koschman.

Now nearly seven years after Koschman was punched on April 25, 2004, Chicago police have decided to re-investigate his death.

The case was reopened after the Chicago Sun-Times filed a request under the Illinois Freedom of Information Act to obtain all police reports on the death.

Police refused to release any records, except a heavily-redacted crime scene report that gives little but??? Koschman’s name along with the date and location where he was hit.

But the Sun-Times own reporting raises troubling new information, including the disclosures that Vanecko and two of his friends initially fled the scene and Koschman’s friends insist their friend was the victim of a “sucker punch.”

Also, the Cook County State’s Attorney’s Office stands by its initial conclusion in the case — but says the case file has mysteriously disappeared.

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The original police report sheds little light on the case, so much information was blacked out before it was released to the Sun-Times.

“The release of the deleted information which includes, but is not limited to crime scene details, witness and suspect names and statements would interfere with the department’s ongoing criminal investigation,’’ according to Officer Rory R. O’Brien, the police department’s Freedom of Information officer. O’Brien’s letter to the Sun-Times is dated Jan. 18, one day after detectives re-interviewed Koschman’s four Rush Street companions.

The Sun-Times has uncovered numerous facts and details that raise questions about how the police and prosecutors handled the investigation that they apparently didn’t start until Koschman died, nearly two weeks after he was listed as a battery victim.

These are some of the Sun-Times’ findings:

■ Vanecko and a friend, Craig Denham, ran away after Koschman was punched in the face. It’s unknown where and when police found them. Another friend ran, but was stopped.

■ Vanecko refused to give police a statement, while his three friends agreed to talk to police.

■ Vanecko appeared in a police lineup nearly a month after Koschman was punched. But none of Koschman’s four friends or another witness could pick Vanecko out of a lineup.

■ Denham, now a brother-in-law of the mayor’s daughter Nora Daley Conroy, and McCarthy, whose father-in-law is a close friend of the mayor, also appeared in police lineups.

■ Police say they concluded Koschman was the aggressor because Kohler and Koschman’s friends???? say Koschman charged Vanecko’s group. But Koschman’s friends tell the Sun-Times Koschman didn’t do that, maintaining he was the victim of a “sucker punch.’’

■ The Cook County State’s Attorney’s office, then headed by Richard Devine, a close ally of the Daley family, determined there was no evidence to charge anyone with Koschman’s death, so his staff closed the case.

Today, the state’s attorney’s office is unable to find any files on the case

A spokeswoman for Devine’s successor, Anita Alvarez, said they still consider the case closed despite the police department’s renewed investigation.

This is all upsetting to Koschman’s mother, Nanci, who says she has a hard time believing police are re-examining her son’s death since they have not spoken to her since May 2004.

“I’m angry at the people who took his life. I want someone to say I’m sorry. I’m realistic I’m never going to get that.

“He was murdered. A person punched him. Whether they intended it to be the final blow, it turned out to be the final blow.’’

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David Koschman almost didn’t go to Rush Street that April night.

A nut for the Cubs and stock cars, he was debating whether to join some friends at a NASCAR race or go with another group for a night of drinking on Rush Street followed by a Cubs game on April 25.

He decided to go downtown.

“I loved the city,’’ said his mother, who grew up on the Northwest Side. “I took him to Bulls games, Cubs games.”

David Koschman was the only surviving son of Nanci and Robert Koschman. He had an identical twin who died during pregnancy, and Nanci Koschman gave birth to her son David and a stillborn child on Feb. 12, 1983. The doctors told her she could have no more children.

Since David was a twin, his mother thinks it may have affected his stature.

“He was very slight,’’ she said. “He was 5’5’, well I think he might have been 5’4.’’

The Koschmans raised their son in a small, ranch home in Mount Prospect. He played baseball, and made lifelong friends.

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Life changed on Sept. 24, 1995, when David’s father died suddenly from a heart problem. David was 12 years old.

He helped his mother around the house, cutting the grass, shoveling the snow. “He didn’t replace my husband, but we were very close,’’ Nanci Koschman said.

David Koschman was “very friendly, nice, like a jokester,’’ said Lela Bork, a classmate from Prospect High School. “He was really, really close to his mom. There were nights when we would go out and he would go to a movie with his mom.’’

He was “never any trouble . . . a very spirited young boy, a lot of personality,’’ recalled Pat Tedaldi-Monti, the high school’s dean. “Absolutely not an aggressive boy at all when I knew him in high school.’’

David Koschman graduated from Prospect High in 2001, along with the four good friends.

Two of them — Scott Allen and James Copeland — worked with David Koschman at an insurance company in Arlington Heights. Koschman was also taking classes at Harper Community College and planned to transfer to Roosevelt University.

Nanci Koschman thought nothing of it when the five friends decided to sample Rush Street on that April night in 2004.

“He learned about Rush Street through me and all my friends, saying that’s a rite of passage,” said Nanci Koschman. “I wasn’t a mom who says, `Don’t go to Rush Street, you’re going to die there.

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He didn’t die on Rush Street.

That happened 12 days later, from the brain injury he got when his head hit the street.

Here’s what happened on Saturday, April 24, 2004, according to his four friends, Scott Allen, James Copeland, Dave Francis and Shaun Hageline.

Vanecko has not responded to requests for an interview, while his companions have declined comment.

That evening, Koschman, Allen, Copeland and Francis drove down to Hageline’s apartment near California and Milwaukee, where they planned to spend the night, The five of them went out for the night, making their way to Rush Street. They hit a few bars, and ended up at Bar Chicago, now known as Detention, a nightclub on the second floor at 9 W. Division.

At 3 in the morning, they left the bar.

“We were just walking down the street,’’ Allen said. “Dave brushed shoulders with some guy. They were drunk. We were drunk as well. Words were exchanged, f-you, screw you. . .”

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Koschman and his friends didn’t know the people they bumped into, all of whom have powerful political connections:

■ Vanecko, then 29, who is named after his late grandfather, Mayor Richard J. Daley. He’s the youngest son of Mary Carol Vaencko, a sister of Mayor Richard M. Daley. Back in 1992, the mayor’s son Patrick and Richard Vanecko pleaded guilty to misdemeanor criminal charges from a brawl at the mayor’s Michigan home where a teenager was beaten with a baseball bat. Vanecko, then a senior in high school, held a loaded shotgun during the beating, court records show.

■ Denham, also 29, a LaSalle Bank official. He later married the sister of Sean Conroy, the mayor’s only son-in-law.

■ McCarthy, then 31, and his wife Bridget, then 26. Her father, Jack Higgins, is a friend of the mayor and a well-known developer who built the city’s police headquarters.

“They were arguing,’’ Hageline said. “We were trying to break it up, but it kept going. Dave was a smaller guy. He wasn’t really letting it go. He was being kind of mouthy.

“The guy he was arguing with was bigger than him, and bigger than me. At least 6’1”, 6’2”. He was definitely the biggest guy in the group.’’

At 6 foot, 3 inches, Vanecko was the tallest in the group, according to Illinois driving records.

“The next thing I know — it was pretty much a cheap shot, really — the guy just clocks him,’’ Copeland said. “There was nothing physical of any kind until this guy throws a punch.’’

Koschman, who appeared to be knocked out, fell backwards off the curb, striking his head on the street. The three older men — including the one who hit Koschman — fled, apparently south on Dearborn, according to a police report and the four friends of Koschman.

Allen said he chased down one of three men, and tackled him in front of a police officer. The Sun-Times has since learned that man was McCarthy, who remained on the scene with his wife.

A fire department ambulance took Koschman to the closest hospital, Northwestern Memorial, where he died 12 days later. The medical examiner’s office determined he died of “crainiocerebral injuries due to blunt trauma. The manner of death is classified as homicide.’’

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After Koschman’s death was ruled a homicide, Chicago detectives began interviewing people who had been on Rush Street. Those witnesses included Phillip Kohler, who first talked to police the night Koschman was hit.

“I saw a group of guys kind of arguing,’’ Kohler told the Sun-Times recently. “There was a kid on the outside. He got really aggravated. I think somebody said something to him. He started jumping up and down. He fell backward and hit his head on the curb. I didn’t see the punch. It seemed like a punch.’’

Kohler told police he didn’t recognize anyone involved in the argument, which he and a friend happened upon after leaving a Rush Street Bar. Police had him view a lineup on May 20, but he said he was unable to identify the man who threw the punch.

Shortly thereafter, news reports disclosed Vanecko’s involvement. Kohler said he then remembered that he and Vanecko went to the same high school, and had “a couple classes’’ together — something he says he never told police even when he was re-interviewed last month.

“I’m surprised the police never figured that out,’’ Kohler told a Sun-Times reporter. “How did you figure out that we went to Loyola [Academy] together?”

Koschman’s friends also viewed lineups on May 20. Some of them picked out McCarthy and Denham, but none of them could identify Vanecko.

One of Koschman’s friends told the Sun-Times that his friend was hit by a guy wearing a hat, but he says no one wore a hat in the lineup. Vanecko is bald.

“I didn’t even understand why there was a lineup,’’ Hageline said. “They knew he [Vanecko] was on the scene. They caught his friend [McCarthy] on the scene.’’

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Ron Yawger, the Chicago police detective who investigated Koschman’s death in 2004, said there were no criminal charges filed because no one could finger the man who punched Koschman.

“I was shocked nobody picked him out of a lineup,’’ Yawger said. “There was nobody who could say he was doing the striking. What you feel in your heart and what you can prove are two different things.’’

Yawger said “Everybody was drunk. Everybody involved was plastered.”

Phil Cline, who was then Chicago’s police superintendent, said last week that no charges were filed in Koschman’s death because he was the aggressor. “At the best, it was mutual combatants. If the other person is the aggressor, then Vanecko has the right to defend himself.’’

Prosecutors agreed, according to this statement Cook County State’s Attorney Anita Alvarez issued Wednesday: “All witnesses who were questioned indicated that Koschman was the aggressor and had initiated the physical confrontation by charging at members of the other group after they were walking away.”

Alvarez was a top prosecutor under Devine. Her office now is unable to find the case file for Koschman’s death.

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Koschman’s friends are dumbfounded that police and prosecutors believe Koschman physically threatened anyone that night, saying he never threw a punch.

“Self defense? How could that be? My friend would have to have initiated some kind of physical threat — and that never happened,’’ Copeland said.

“Someone was killed that night and there was no punishment at all for it. If that punch wasn’t thrown, we wouldn’t be talking about this right now.’’

Whoever threw that punch could be charged with first degree murder, according to Richard Kling, a criminal attorney and law professor at IIT Chicago-Kent College of Law.

“If I were a prosecutor, one punch is enough for a murder charge,’’ Kling said. “ He probably didn’t intend to kill, which brings it down to involuntary manslaughter. My argument would be that a 6-foot, 3-inch guy knows that a punch to the head can cause great bodily harm.’’

Koschman is still upset about the police investigation.

She says the police never contacted her while her son was unconscious in the hospital or even after he died.

She decided to call the police after reporters started knocking on her door, asking about Vanecko’s role in her son’s death. A detective agreed to meet with her, giving her a largely blacked-out police report and little other information.

“He more or less said David started the fight,’’ she said. “I wasn’t allowed to see the other names on the report. But I was told I would be very impressed by the names of the people who were involved.”

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