MOUNT VERNON, Ill. --Police chief Tim Mendenall in Mount Vernon suspected serial killer Timothy Krajcir gave a bogus confession last month to a 1981 knife attack that sent another man to prison.

Now the chief thinks he knows where Krajcir got some of the details.

The Illinois Department of Corrections on Thursday confirmed that Krajcir and Grover Thompson -- the man convicted of attempted murder in the assault of 72-year-old Ida White -- both lived at the same Illinois prison's psychiatric ward for years before Thompson died behind bars in 1996.

Mendenall believes Thompson may have casually discussed the White case behind bars with Krajcir, who could have absorbed some of the details and passed the story off as his own when he confessed last month in the case, along with nine old killings in four states.

A Carbondale, Ill., investigator who took those confessions remains convinced Krajcir is White's assailant, raising speculation that Thompson died an innocent man in prison.

But Mendenall points to "very, very strong evidence" against Thompson that convinced a jury of his guilt -- and voids in Krajcir's account that he easily should have recalled if he committed the crime.

"This kind of hurts Krajcir's credibility" about his confessions to killings in Illinois, Missouri, Kentucky and Pennsylvania in the late 1970s or early 1980s, Mendenall said. "Is he telling the truth about everything, you know?"

In Carbondale and Cape Girardeau County, where Krajcir was charged in five killings last month, authorities firmly believe Krajcir isn't a liar.

"All I can say is he has told the truth in everything else. I can't rationalize why he would lie," Carbondale police Lt. Paul Echols said this week.

Cape Girardeau County prosecuting attorney Morley Swingle said DNA evidence further ties Krajcir to two of the killings there, with other evidence in the remaining three slayings corroborating the inmate's accounts.

Krajcir, 63, is likely to die in prison, saddled in the past several weeks with back-to-back, 40-year prison sentences in the 1978 slaying of a 51-year-old housewife in Marion, Ill., and the 1982 rape and strangling of a student at Southern Illinois University in Carbondale.

Krajcir next is to be prosecuted in Missouri, then face Pennsylvania charges in the 1979 rape and slaying of a 51-year-old woman and Kentucky kidnapping and burglary counts related to the killing of a 29-year-old woman.

Krajcir's confessions to the old killings were given in exchange for assurances he would not face the death penalty.

In Mount Vernon, about 98 miles north east of Cape Girardeau, White said the intruder attacked her in her bathroom, slashing her with a pocketknife when she wouldn't stop screaming. She described him as black, as did a neighbor who interrupted the attack and tried to keep the suspect from escaping through the window.

Krajcir is white.

Mount Vernon police found Thompson, a black man, near the post office across the street from White's house within a half hour of the attack, a bloody knife in his pocket.

In his confession, Krajcir described "in great detail" how he was peeping through White's windows when he came across an unlocked bathroom window, then climbed in and hid in a shower stall before he ambushed the woman when she entered the bathroom, Echols said.

"He said he grabbed her, and she wouldn't stop screaming so he subsequently stabbed her and fled out the window," Echols said, calling the scenario consistent with Krajcir's methods of sneaking his way into women's homes.

Mendenall isn't buying it.

White, now deceased, and her neighbor each identified Thompson as the intruder, he said. Krajcir never mentioned the neighbor kicking in White's door and scrambling to the screaming woman's rescue, then ripping the suspect's shirt in the confrontation. When arrested, Thompson's shirt was torn.

Krajcir told Echols the woman might have been in her 50s, maybe older. White was 72.

And Krajcir said White's bathroom had no shower curtain; at trial, the woman testified her attacker tried to keep her from peeling it open.

Mendenall suspects Krajcir may have lied because he has nothing to lose.

"I don't know the man. I've never spoken with him," the chief said. "But it matters not what he claims he's done. He's never going to get out of prison. Whether he confesses to 30 homicides or one, he's not getting out of jail."