Levi Bellfield has admitted abducting, raping and killing schoolgirl Milly Dowler for the first time, Surrey Police have said.
In 2011, Bellfield, 47, was found guilty of the 2002 murder at the Old Bailey and sentenced to life in prison.
Thirteen year old Amanda "Milly" Dowler was abducted on the way home from school in Walton-on-Thames, Surrey in March 2002.
Her body was discovered six months later nearly 25 miles away in a wood in Hampshire.
Due to the level of decomposition, experts were unable to tell how she died.
The former nightclub bouncer is believed to have made the admission during an investigation into whether he had an accomplice.
A Surrey police spokesman said a man in his 40s was arrested on Wednesday "in relation to allegations of rape and assisting an offender" but later released without charge.
Bellfield, who now calls himself Yusuf Rahim, had already been found guilty for the murder of two women, Marsha McDonnell and Amelie Delagrange, and the attempted murder of Kate Sheedy in 2008.
He was also accused of attempting to abduct 11-year-old Rachel Cowles the day before Milly's disappearance.
Ms Cowles' case collapsed after tabloid newspapers published reports about the guilty verdict for Milly's murder which contained prejudical information before the jury had reached a verdict on the attempted kidnap.
The detective who investigated the murders of Ms McDonnell in 2003 and Ms Delagrange in 2004 said he was "surprised" Bellfield confessed.
DCI Colin Sutton told ITV News Bellfield had defended himself against the Milly charges "tooth and nail" and put the Dowler family "through an extremely difficult time as a result".
In February 2012 Bellfield lost a bid the challenge his conviction at the Court of Appeal in London.
In a statement issued by their solicitor, the Dowler family said they had been made aware of the additional arrest but asked for their privacy to be respected.
It said: "They have been aware for some considerable time that a new police investigation was taking place into the circumstances of the abduction and murder of Milly.
"The effect of this information has been devastating for a family which has already had to endure so much."
The police spokesman said: "The arrest was made following new information regarding an accomplice being involved with Levi Bellfield (who now calls himself Yusuf Rahim) in the abduction and rape, but not murder of Milly Dowler.
"Police have thoroughly investigated this claim and as the final part of their inquiries made the arrest today. The man arrested has subsequently been released with no further action as no evidence has been found to support the allegations."
Additional reporting by PA
