GRAND RAPIDS, Mich. -

A federal jury has found a man guilty of sexually assaulting and murdering his 3-month-old daughter on a northern Michigan Indian reservation.

The U.S. District Court jury in Grand Rapids reached the verdict Wednesday against Steven Deuman.

The first-degree murder conviction carries a mandatory penalty of life in prison without parole.

The girl died Aug. 12, 2011, on the Grand Traverse Band of Ottawa and Chippewa Indian Reservation, northwest of Traverse City.

An autopsy indicated the baby was asphyxiated.

Defense lawyer Richard Stroba says the death was accidental, and Deuman wouldn't harm his children.

Federal authorities investigated the death because it occurred on tribal land.

Deuman and the baby's mother lived on the reservation.

During the trial, witnesses testified that Deuman sexually abused them when they were children.