A former detective has said three investigations into Cyril Smith sex abuse allegations were stopped.
Retired Det Sgt Jack Tasker, who served with Lancashire Police, carried out the third investigation, following two by the former Rochdale Borough force.
He claimed senior officers ordered him to hand over notes and warned he would be "in serious trouble" if he continued the investigation.
He believes someone "high up" made sure the case never got to court.
Mr Tasker said he was asked in 1969 to investigate the allegations by officers from Rochdale, whose own inquiries into abuse at the privately-run Cambridge House care home in the town had been halted.
'We believed them'
"They had twice investigated Cyril Smith for paedophilia and each time the chief constable of Rochdale [the late Patrick Ross] had taken the file away and told them to stop," he said.
"They suggested that myself and [my partner] could get into Rochdale and get around these people without anyone getting to know.
"They gave us the names of the boys and we interviewed them and eventually, with a solicitor, we interviewed Cyril Smith at the police station."
He said he believed the evidence given by the boys, who had been residents of the home.
"There was no DNA in those days - you had to believe the victims when they told you, and, quite frankly, myself and my colleague did believe them," he said.
Later he interviewed Smith at a police station in the presence of his solicitor.
"He was very, very nervous, and his last words when he left with his solicitor was, 'This will kill my mother'," he said.
"We concluded that he had a case to answer and I think those boys would have stood up in court.
"My opinion of Cyril Smith was that he was a big bumptious bully and any decent barrister or solicitor would have made mincemeat of him."
The same opinion was voiced by a detective superintendent in a 1970 report to the chief constable of Lancashire as part of a fourth investigation.
This got as far as the Director of Public Prosecutions, who decided not to proceed.
'Scratching the surface'
Later Mr Tasker said two senior officers came to his office and told him to hand over "every scrap of paper" on the investigation.
"[They said] we must keep out of Rochdale and we must not speak to anyone about the Cyril Smith inquiry or we'd be in serious trouble," he said.
He believes many other men were involved in the abuse and the Smith investigations were only "scratching the surface".
"It was obvious that someone quite high up, quite powerful, was making sure that Cyril Smith never appeared before a court."
Smith's family said he always denied such accusations and said they were saddened that allegations were now being made when he could no longer defend himself.
The Smith family said they would continue to co-operate with any further investigations.