A homeless man pleaded not guilty Monday to first-degree murder in the death of his 6-year-old cousin, who was found naked and wrapped in a blanket inside an SUV after the man allegedly led police on a high-speed chase.

Ryan Bois, who was wearing a hospital gown, spit at reporters and photographers as he was escorted to his arraignment in Quincy District Court.

Bois, 20, pleaded not guilty to murder, assault and battery with a dangerous weapon against a police officer, multiple motor vehicle violations and possession of heroin at the time of his arrest. He was ordered held without bail and will undergo a psychiatric evaluation at Bridgewater State Hospital.

Police found the body of Joanna Mullin wedged on the backseat floor of the vehicle Bois drove when he allegedly led police on the chase through Weymouth and Quincy early Sunday. After he crashed the white Ford Explorer into a cab and fled on foot, he threatened pursuing police with a knife, authorities said.

The girl was naked and wrapped in a comforter in the SUV, which was stolen from her grandmother's house, Norfolk Assistant District Attorney Michael Connolly said.

Mullin was at her grandmother's house in Weymouth for a sleepover with another cousin when Bois allegedly climbed on top of the SUV and cut open a screen to enter the house in the middle of the night, Connolly said. The grandmother, who was in an adjacent room, did not know Mullin was missing before she was contacted by police.

The cause of the girl's death has not been released, but Connolly said she was sexually assaulted. Mullin had severe trauma to her face and bruises on her body and was bleeding from her mouth, but it was not clear exactly where or when she died, he said. She did not die in the crash.

Investigators found bloodstains and fluid consistent with semen on sheets at the grandmother's home, Connolly said.

After his arrest, Bois was treated at South Shore Hospital in Weymouth for minor injuries. He will undergo a competency and criminal responsibility evaluation at Bridgewater State Hospital, and will return to court Aug. 24. He could still face other charges, Connolly said.

Bois had an outstanding warrant for violating probation terms, which stemmed from robbery charges, Connolly said.

Calls to Bois' attorney, Beverly Cannone, were not immediately returned.