Elkhart, Ind. ? Police and federal agents extended their search to Chicago on Sunday for four children and their mother, whose ex-boyfriend authorities say shot a man and kidnapped them in Indiana the day before.

Authorities have issued an Amber Alert and said the children and their mother, 31-year-old Kimberly N. Walker, were in extreme danger. Walker and Jerry D. White, 30, the father of the four children, once lived in Chicago, and White has family there, police said.

Police said they recovered two cars, a Dodge Intrepid that White used to flee and Walker’s two-door Saturn, which White is suspected of stealing Friday. The cars were found Saturday night in Elkhart, about 90 miles east of Chicago.

The cars have not provided additional clues to investigators, Elkhart police Lt. Peggy Snider said Sunday. Police reported no new developments late Sunday afternoon.

An arrest warrant with charges of attempted murder and several counts of confinement has been issued for White, said Detective Sgt. Bill Wargo.

Police said that White had apparently been harassing Walker over the past few days, and that her sister and her sister’s boyfriend had decided to stay with her and the children. Walker called police Friday and said White had confined her in her car, the Saturn, before stealing the vehicle, authorities said.

About 2 a.m. Saturday, White broke into Walker’s house and shot her sister’s boyfriend, Wargo said.

Police said White held everyone hostage until about 11:30 a.m., when he fled with his five hostages in the Dodge, which belongs to Walker’s sister. Walker’s sister was then able to call police.

Her boyfriend, whose name was not released, was in serious condition Sunday, police said.