Chelsea midfielder Mikel John Obi's contract with former club Lyn Oslo is being investigated by Norwegian police.

The 19-year-old Nigerian midfielder moved to Stamford Bridge from Lyn despite having already agreed a switch to Manchester United before the clubs agreed a financial deal to allow him to join the Blues.

Norwegian officers are investigating claims of contract forgery although it is understood the probe does not involve Chelsea or United.

"The case is about document forgery and that is serious," Oslo police attorney Paal Hernaes said.

Hernaes, who reportedly revealed Mikel was questioned last week, added: "Mikel is just a witness in the case."

Lyn claim now-departed sporting director Morgan Andersen dealt with the matter.

A spokesman for the club told The Guardian: "We know there have been police investigations, but we haven't heard anything from the police and, as far as we are concerned, the matter has nothing more to do with us.

"It was Morgan Andersen who dealt with it and he has now left the club."