A woman involved in a custody battle killed herself by stepping in front of a train, and authorities later found her 7-year-old daughter dead at her home.

The girl had been suffocated, authorities said.

"We believe that this could be a murder-suicide situation," McHenry County Sheriff Keith Nygren said Wednesday.

Authorities say Magdalene Kamysz, 28, stepped into the path of a commuter train Tuesday afternoon, then turned her back just before it struck and killed her.

A deputy coroner went to her home in this Chicago suburb to break the news to family members. While searching for clues to the suicide, he found the body of Sydney Kamysz in a locked bedroom, Nygren said.

Sydney, described as a bright girl fluent in both Polish and English, was about to enter second grade at a Crystal Lake elementary school.

Sydney's 29-year-old father, Alan Burton of Walworth, Wis., filed paperwork in February seeking joint custody, according to court records.

"It's been somewhat heated and contested," Nygren said. In June, a judge had ordered the battling parents, who were not married, into mediation.

Kamysz's lawyer, Wesley Pribla, denied that custody negotiations had been unusually contentious and said he did not see any indication his client might do something drastic.

"She impressed me as being a loving mother who had nothing but the best interests of her daughter at heart," Pribla said.

No suicide note was found, Nygren said.