The evidence came out on the first day of a probable-cause hearing to decide whether Sean M. Phillips, 24, should face an open murder charge in Mason County Circuit Court in Ludington.
Authorities say he killed Katherine Phillips in 2011, but they've never found her body. Sean Phillips already is serving 10-15 years in prison for unlawful imprisonment in the girl's disappearance.
Authorities said he wrote the letter from prison in 2012 to the baby's mother, Ariel Courtland.
In the letter, Phillips said he violently ripped Katherine Phillips' car seat from his car and she "was thrown from it."
He said he held the baby's body and cried, then drove around for a long time before setting the infant down in a "peaceful place." He said he didn't intend to leave the body, but got lost and couldn't find it.
Courtland was Thursday's first witness, talking about the last time she saw her daughter.
Under cross-examination by defense lawyer David Glancy, she said that she didn't believe Phillips would deliberately hurt their child.