WASHINGTON (AP) — Federal authorities have opened an initial criminal inquiry into whether Rep. Gary Condit obstructed justice or encouraged perjury in the investigation of Chandra Levy's disappearance, law enforcement officials said Thursday.

Authorities specifically are looking at a flight attendant's claim that Condit urged her to sign a statement denying a 10-month affair she says they had. Anne Marie Smith also said Condit told her she did not have to cooperate with FBI agents who questioned her.

"It is in the preliminary stages, talking to witnesses and trying to determine if we should proceed further," said one law enforcement official familiar with the matter, who spoke only on condition of anonymity.

Condit has denied asking anyone to lie or not cooperate with investigators who are trying to figure out what happened to Levy. He has not commented on any relationship with Smith.

Marina Ein, a spokeswoman for Condit, repeated Thursday that he is cooperating with police in all aspects of the investigation.

Smith, 39, was interviewed Wednesday and again Thursday by FBI agents and prosecutors from the U.S. attorney's office.

Law enforcement officials said the preliminary inquiry is routine when such allegations surface but prosecutors decided to join FBI agents in the initial interview of Smith because of the congressman's profile. "What is on the plate is perjury and obstructing justice, and we must decide if these allegations have merit," one official said, speaking on condition of anonymity.

The search for Levy, a former federal intern, was proceeding on several fronts Thursday, including a search of vacant buildings in the nation's capital.

"We have to explore the possibility that she may be dead and we're looking for the remains," said Terrance Gainer, the city's No. 2 police official. "So we're looking at about one-seventh of the District of Columbia right now, at abandoned buildings.

"Our officers begin walking through those buildings today and we may use cadaver dogs if it's necessary," Gainer said on CBS' "The Early Show."

Gainer said investigators still are working on four theories in Levy's disappearance: that she was a victim of foul play, committed suicide, walked away voluntarily or is wandering around not knowing who she is.

Police also are negotiating with Condit's lawyer, Abbe Lowell, to set conditions under which the California Democrat would submit to a lie detector test. Condit, who police say is not a suspect in Levy's disappearance, told authorities last week that he had a romantic relationship with the 24-year-old woman, police sources say.

"The congressman's attorney has suggested we can get to a polygraph" test, Gainer said.

The Washington Post and CNN, citing police sources, reported police have asked three other men to take polygraph tests. One said he occasionally socialized with Levy, but the connection of the other two to Levy was not immediately clear.

Several items taken from Condit's apartment during a search that ended at 3 a.m. Wednesday were being turned over to the FBI crime lab in Quantico, Va., said Sgt. Joe Gentile, a police spokesman.

Condit voluntarily allowed the search to show his cooperation in the search for Levy, a constituent from Modesto, Calif., whom he has described publicly as a good friend.

Meantime, the Post reported in Thursday's editions that FBI agents have interviewed a Pentecostal minister who described an affair between his then-18-year-old daughter and Condit, who is 53 and married with two grown children.

The minister, Otis Thomas, was quoted by the Post as saying Condit had told his daughter never to speak of the relationship.

On Thursday morning, a handwritten note was posted on the door of Thomas' home in Ceres, Calif. It was signed "Jennifer Thomas" and said, "I never met that congressman who's involved in all this. ... I don't even know how both me and my father got mixed up in this, we don't know anything."

Betty Hoffman, who lives below the Thomases, confirmed Jennifer Thomas is Otis Thomas' daughter. Hoffman said she last saw Otis Thomas three days ago, and he told her the FBI suggested he leave town to avoid the media.

Thomas told the Post his daughter is afraid to talk with the FBI and has gone into hiding, but Hoffman said Jennifer Thomas works at a local fast food restaurant and was home Wednesday.

Thomas told the Post that his daughter met Condit at a political rally and ended the relationship about seven years ago. Thomas, whose church is in Modesto, did gardening work at the Levy home in Modesto, the Post said.

Asked about that report, Gainer would say only that "the names and stories aren't unfamiliar to us."