British police said they found the remains of a 2-month-old baby in woodland Wednesday after officers arrested the infant's mother and her boyfriend, who vanished after the child was born in early January.

Constance Marten, 35, and her boyfriend Mark Gordon, 48, a convicted sex offender, had crisscrossed England for weeks to avoid authorities, police said. Investigators suspect the pair used large amounts of cash to live off the grid, traveling around the country by taxi, covering their faces when near closed circuit cameras and moving frequently, often after dark.

They were detained Monday in Brighton, southern England, after a tip came from a member of the public. The baby was not with them.

The couple was initially arrested on suspicion of child neglect, and the grounds were upgraded Tuesday to suspicion of gross negligence manslaughter.

"A crime scene is in place, and work at the location is expected to continue for some time," Detective Superintendent Lewis Basford of London's Metropolitan Police force said in a statement. "This is an outcome that myself and the many officers who have been part of the search had hoped would not happen."

An autopsy will be performed to determine the cause of the infant's death, Basford said. Marten and Gordon remained in custody after police applied for a 36-hour extension of their detention period.

Before the remains were found, police had expressed concern that serious harm had come to the baby. Dozens of police officers, assisted with a helicopter and drones, searched for the child in woodland and open areas near where Marten and Gordon were detained. They also urged local residents to look in their outhouses and sheds.

Police launched a national search for Marten and Gordon after a car in which they were traveling was found in flames on a roadside on Jan. 5.

Police believe Marten gave birth in or near the car a day or two earlier. In the weeks before she and Gordon surfaced, officers said they were concerned for the family's welfare because neither Marten nor the baby had received medical care.

Marten, who is from a wealthy, aristocratic British family, was reportedly a drama student when she met Gordon. He served 20 years in prison in the United States after being convicted in Florida of kidnapping and sexual battery, according to U.S. law enforcement records. He was deported from the U.S. after his release.

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