Prosecutors still aren't sure what happened to 6-month-old Lexi Tucker, who died last year of blunt force trauma to her back.

But they did determine that the death happened while she was with her father, Darnell A. Harper Jr.

The 20-year-old Homewood man pleaded guilty to third-degree murder yesterday as part of an agreement with prosecutors not to pursue a first-degree conviction. He is scheduled to be sentenced by Common Pleas Judge John A. Zottola on Feb. 18.

Since Lexi's death on April 3, 2007, Mr. Harper has given several different stories to investigators, none of which reveal how the child was injured.

First, he told police a friend he referred to as "Blue" came over and they smoked marijuana together. Mr. Harper said he left Lexi alone with Blue for less than two minutes and then discovered that the child wasn't breathing.

Mr. Harper later backed off that account and said he took the child to a Wilkinsburg bus stop and while waiting for a bus, he picked her up and she felt limp in his arms.

Investigators did determine that Lexi's mother, Ashlei Tucker, left the child in Mr. Harper's care in the morning so she could go to work. When he discovered the child was not breathing, Mr. Harper called Ms. Tucker. After the mother returned home, they called for an ambulance.

Lexi was pronounced dead at Children's Hospital and an autopsy revealed lacerations to the child's kidney and liver as well as broken ribs -- some recent, others not.

Regardless of how the child was injured, prosecutors filed charges because Lexi was in Mr. Harper's custodial care when they occurred.