The investigation, codenamed Operation Enamel, was launched by Leicestershire Police in early 2013 and led to the former city MP being charged with sexual offences against children over several decades.
However, the evidence the Operation Enamel officers gathered from interviews with more than 100 alleged victims and witnesses was never tested in court as Lord Janner died aged 87 in December 2015.
The veteran Labour figure had been due to go on trial in February 2016 accused of 22 sexual offences against nine people, most of whom had been living in children’s homes at the time of the alleged abuse.
Matt Hewson, a senior detective who worked for Leicestershire Police for 35 years, led Operation Enamel.
He gave evidence at the Independent Inquiry into Child Sexual Abuse, (IICSA), yesterday on the second day of its investigation into whether Lord Janner received ‘preferential’ treatment by police and other public bodies because of his social standing.
Mr Hewson said his team also reviewed previous inquiries conducted by the force into allegations against Lord Janner.
Those inquiries did not lead to criminal charges against the politician – although a review by a retired High Court judge several years later ruled that had been a mistake.
Mr Hewson was questioned about the scale and nature of the investigation yesterday by Jacqueline Carey, counsel to the inquiry team.
Mr Hewson said: “Initially, the numbers were minimal and there were three officers taking part in Operation Enamel.
“As that investigation progressed into the summer and autumn of 2013,additional officers were appointed, including myself.
“There were 46 officers used throughout Operation Enamel, although at any one time there would be 30 working on the investigation.
“Thirty officers is a large inquiry. You would expect smaller numbers if you were investigating a high category homicide, for example.”
Asked by Ms Carey about the cost of the investigation, he said: “It’s hard to tie down exactly, but by the time January 2016 it came to approximately £400,000.”
Further complainants came forward to make allegations against Lord Janner as the Operation Enamel team continued their work, Mr Hewson said.
He added: “The scale was immense.”
Following Lord Janner’s death, an official report by a retired High Court judge concluded that three chances – in 1991, 2002 and 2007 – to put the former Leicester West MP on trial were missed because of mistakes by the police or prosecutors.
Lord Janner was accused of multiple sexual offences against boys between 1963 and 1988 but died shortly before Christmas 2015. He had been suffering from dementia.
His family insists he is innocent of all allegations.
The Independent Inquiry into Child Sexual Abuse (IICSA) is investigating how public bodies handled the allegations against Lord Janner, who is said to have committed acts of abuse in children’s homes, schools, a flat in London and in Parliament itself.
The hearing, parts of which are taking place in private session to protect alleged victims’ names, is continuing and expected to sit for three weeks.