A cold case team is looking into allegations that police released the former Liberal MP Cyril Smith after child pornography was found in the boot of his car when he was arrested in Northamptonshire in the 1980s.
The allegation was made in a book written by the Labour MP Simon Danczuk, which was published last year.
Mr Danczuk claims that Smith, who died in 2010, was stopped on the M1 in the county on suspicion of a driving offence about 30 years ago.
But he was then released from police custody after making a phone call to an unidentified person in London.
The child pornography said to have been found in his boot disappeared, Mr Danczuk said.
Assistant Chief Constable Ivan Balhatchet, of Northamptonshire Police, said today (Thursday): “We are aware of the allegations that have been published in Simon Danczuk’s book and in the newspapers.
“Detectives in Northamptonshire Police’s Cold Case Team are currently making inquiries to establish the nature of the circumstances surrounding the disclosures made in the book.
“We would encourage anyone who knows anything about the claims in the book relating to alleged events in Northamptonshire to come forward and tell us.”