Secret Service probes threats on Barack Obama — police also investigate swastikas, KKK symbols, racial slurs near Ann Arbor, Mich.

HOWELL, Mich. — The U.S. Secret Service is investigating two letters threatening Democratic presidential nominee Barack Obama.

Douglas Zloto, resident agent in charge of the federal agency’s Saginaw office, said authorities are trying to determine the nature and extent of the threats. One letter was sent to the Livingston County Democratic Party and another to a local business.

‘‘The Secret Service takes every threat extremely seriously,’’ Zloto told The Ann Arbor News. ‘‘We will be using every resource to investigate this threat ... and to determine how it was made.’’

The letter sent to the party office reportedly expresses a wish that Obama ‘‘gets a bullet.’’

The second to the unidentified Howell business owner includes racial slurs — preceded by a warning that the owner should ‘‘think hard about who you are trying to elect.’’

Michigan State Police said they are investigating racial slurs spray-painted on an Obama campaign billboard along U.S. 23 near Ann Arbor. Officers believe the graffiti, which includes swastikas and Ku Klux Klan symbols, were painted late Tuesday or early Wednesday.

Obama state campaign spokesman Brent Colburn said he believed it was the first large-scale act of vandalism on Obama campaign material to include such slurs in Michigan.

‘‘Whoever did it should be embarrassed and ashamed, but unfortunately, (racism) is a reality in our world,’’ he said. ‘‘It in no way is a reflection of the reception we’ve gotten in the Ann Arbor area, or anywhere in Michigan.’’