A man in his 20s has been arrested after the skeletal remains of his former girlfriend were found in his home in Kawasaki, investigative sources said.

Hideyuki Shirai, who had been warned by the police for stalking 20-year-old Asahi Okazaki before she went missing in December, returned to Japan from the United States on Saturday afternoon, they said. He left Japan in April.

During a search of Shirai's house on Wednesday, the police discovered a bag containing human remains under the floorboards. They conducted the search on suspicion that the man violated the anti-stalker law.

The police said Saturday that the body had been confirmed as that of Okazaki's. Autopsy findings showed signs of burning and that more than a month had passed since her death.

Okazaki, also a Kawasaki resident, had an on-again off-again relationship with Shirai since she started going out with him from around spring last year. She was being stalked at her workplace and near her home, according to her family.

She went missing around Dec 20 last year while staying at her grandmother's house. She called police nine times in the two weeks before she disappeared.

A window at her grandmother's house was smashed while Okazaki was staying there.

Shirai denied involvement during voluntary questioning by police at the time of her disappearance, the investigative sources said.