Norwegian police are investigating John Obi Mikel's transfer to Chelsea amid suspicions of contract forgery.
The 19-year-old Nigerian midfielder joined the Premiership champions in June 2006 after announcing that he would not be moving to Manchester United from Norwegian side Lyn Oslo as expected.
He had been due to join United in January but said he had been put under pressure to sign for the club and even went missing from Lyn for several weeks at the height of the dispute over his future.
Mikel then came to an agreement with United and Lyn to allow him to move to Stamford Bridge.
"The case is about document forgery and that is serious," Oslo police solicitor Paal Hernaes said.
Hernaes also added that Mikel had been questioned by police in London last week: "Mikel is just a witness in the case," he said.
Former Lyn director Morgan Andersen declined to be interviewed in the case but Mikel's Nigerian agent John Shittu had agreed, Hernaes confirmed.
The police are expected to complete their investigation by early April.