MADISON, Wis. (AP) -- A convicted sex offender has been jailed on a probation hold and is considered a material witness in the death of a University of Wisconsin-Madison student from Marshfield, court records show.

WKOW-TV of Madison disclosed Monday that documents seeking to revoke David Kahl's probation quote him as telling police he broke into a home near 21-year-old Brittany Sue Zimmermann's apartment on the day she was killed and knew two homeless men who were running scams and breaking into houses.

He also helped police develop a sketch of one of the men, the station reported.

Zimmermann's fiance found her body April 2 in their first-floor, off-campus apartment several blocks from the state Capitol. Investigators have said Zimmermann's door was forced open.

Detectives have been targeting Madison's homeless population in their investigation but haven't publicly identified any suspects.

The revocation documents obtained by WKOW said police were treating Kahl as a material witness, not a suspect.

The documents allege Kahl's probation should be revoked because he failed to register as a sex offender, among other reasons.

The documents go on to say, "Mr. Kahl was a material witness in the death of a coed...."

Madison police spokesman Joel DeSpain didn't immediately return messages from The Associated Press late Monday afternoon seeking comment.

DeSpain told WKOW the Zimmermann case was "active" but declined further comment.

Kahl, 42, remained held at the Dane County Jail Monday night, a jail spokeswoman said.

Computerized state court records show Kahl's record includes a second-degree sexual assault conviction in a 1993 case where he pleaded no contest and was sentenced to a year in prison.

(Copyright 2008 by The Associated Press. All Rights Reserved.)