A pregnant woman reported missing from her home in Upper Darby was found dead late Monday, nearly one week after she disappeared, police said. Meanwhile, detectives continued their search for Justin Smith, the woman’s boyfriend and the last person seen with her.

Philadelphia police recovered the body of Dianna Brice, 21, in a wooded area near 58th Street and Eastwick Avenue in Southwest Philadelphia, according to Kevin Ryan, a private investigator working with Brice’s family. Officers found the body about 11 p.m. Monday, and forensic investigators later identified it as Brice’s, he said.

Upper Darby Police Superintendent Timothy Bernhardt on Tuesday said the investigation into her death is being handled by Philadelphia police and is being treated as a murder case.

It remained unclear how Brice died. Sources familiar with the investigation said officers found the body by tracking the young mother’s cellphone.

It was about a mile from where investigators had discovered a car registered to Smith, 23, on fire just hours after her disappearance.

Police have said Smith’s cellphone has been off since March 30, and his wallet and car keys were found discarded near the burning vehicle.

The couple were last seen at a laundromat in Yeadon on March 30, when Smith picked up Brice, according to Brice’s mother, Betty Cellini.

» READ MORE: Upper Darby police searching for pregnant woman who vanished last week with her boyfriend

On March 30, having not heard from her daughter for several hours, Cellini said she called Smith, who told her that the two had gotten into an argument and that Brice got out of his vehicle in West Philadelphia. When Cellini asked for more information and said she planned to file a missing person’s report, Smith became upset and hung up on her, she said.

Cellini declined to comment on the discovery of her daughter’s body Tuesday morning, saying only that she felt numb.

Smith’s father, Michael Kiser, told The Inquirer he had no idea where his son was, and said he was just as surprised about the disappearance as Cellini and her family.

Staff writer Mike Newall contributed to this article.