The estranged husband of missing Illinois mom Lisa Stebic has filed for sole custody of the couple's two children.

Craig Stebic's lawyer said the father is afraid that his wife, who was last seen April 30, may reappear, take the children, then disappear again. He filed an emergency motion on Wednesday seeking custody of the two children, Lexi and Zack, FOX affiliate WFLD in Chicago reports. Craig Stebic's lawyer told FOX News that the move is aimed at protecting the children, but would not elaborate further.

"If indeed Lisa did take off — at this point, we don't know what to think. If she were to come back and take the children with her … Craig is a fantastic father and it would devastate him," Jill Webb, Craig Stebic's sister, told FOX News on Thursday.

Lisa Stebic, 37, last was seen at her home in the 13200 block of Red Star Drive in the southwest suburb of Plainfield, Ill., about 20 miles west of Chicago.

The Stebics were in the midst of a divorce. The couple lived in the same house but rarely spoke.

Police have not named Craig Stebic as a suspect and said he is cooperating. His divorce lawyer so far has advised him to refuse a polygraph test. He maintains he had nothing to do with his

"He told me, 'Dad, I had nothing to do with this,' that's what he told me," Craig's father, Joe Stebic, told WFLD. "I have to believe him; he's my son."

"He assures me he has nothing to do with this at all," added Webb.

Some news reports have suggested that Lisa was fearful of her husband.

When asked about those reports, Webb said: "That is something I had not heard of … he doesn't even know where this is coming from. If Lisa was so afraid of Craig, first of all, why would she stay in the house with him" or leave the kids with him.

Meanwhile, officials were searching lakes near the Stebic home Thursday for any sign of the missing woman. Divers were to enter nearby waters on Friday. Michigan police also have searched a cabin belonging to Joe Stebic.

Plainfield Deputy Chief Mark Eiting told The Chicago Tribune that the FBI was providing aid but there was no indication of the case crossing state lines.

"We asked the FBI to assist us in the investigation," Eiting said. "They have assisted in the search of the residence."

Click here to read The Chicago Tribune story.

Lisa's coworkers at Lincoln Elementary School, where she worked as a cafeteria aid, organized a vigil for her Wednesday night. About 60 people attended and brought with them balloons, stuffed animals and flowers.

Click here to watch WFLD's report on the vigil.

Lisa's children, ages 10 and 12, attended, along with their father.

Craig Stebic said the outpouring of support was "fabulous" as he wiped tears from his eyes.

"We always knew how much Lisa was loved but to see and hear this today, it really means a lot," said family spokeswoman Melanie Greenberg.

Lisa is 5-foot-2, weighs 120 pounds and has long brown hair and brown eyes. She has two visible tattoos, a small rose on her ankle and a large butterfly on her lower back. Anyone with any information is asked to call the Plainfield Police Department at 815-267-7217.

Her family created a Web site, www.findlisastebic.com, to aid efforts to find her and are offering a $20,000 reward for information leading to her whereabouts.

FOXNews.com's Liza Porteus contributed to this report.