WEYMOUTH, Mass. -- A 7-year-old girl kidnapped by her cousin from a home in Weymouth was found dead in a stolen SUV after officers pursued the driver in a high-speed chase, Norfolk County district attorney William Keating said.

Ryan Bois, 20, who was described by police as homeless, drove away after they pulled him over for speeding on Washington Street in Weymouth early Sunday morning.

"What began as an ordinary traffic stop turned into an horrific and tragic discovery," Keating said at a news conference.

The chase started around 1:30 a.m., continued through Braintree and ended when the Ford Explorer collided with a cab near the Quincy shipyard, police said.

A search of the car found Joanna Mullin, 7, dead inside. She was clothed in a nightgown and her body showed signs of trauma.

"It appeared she was wrapped in a blanket. An officer tried to perform CPR," said Tracey Stedman, a dispatcher for Braintree Best Taxi.

Joanna Mullin is Bois' cousin.

Police said they believe Mullin was dead before the crash.

Bois, Keating said, fled from the crash scene, after threatening an officer with a knife. He was apprehended by police officers using a dog in their search.

The young girl was spending the night at her grandmother's house. Bois entered the house, kidnapped her and stole the car, Keating said. Police said he used a ladder to climb into the bedroom that Mullin was sleeping in.

"It was not a random act. The public is not in danger," Keating said. "She was removed from the house unbeknownst to the adult in that house and abducted from that house."

Bois’ mother, Maureen Tofani, told The Boston Herald her son was addicted to alcohol and drugs.

Joanna was the daughter of Tofani's sister Heather Mullin. Mullin and her husband Jerome have two sons, ages 1 and 4, according to Tofani.

“I’m devastated, completely devastated,” Tofani said. “I could see him hurting himself,” she told the paper, “but not those kids. He was really close to those kids.”

NewsCenter 5's Lynn Jolicoeur reported Bois is well known to Weymouth police.

He is scheduled to be arraigned Monday in Quincy District Court on a charge of first-degree murder. An autopsy is being performed to determine the cause of death.