Pressure was mounting on the Sunderland AFC hierarchy to explain when they became aware of the full extent of Johnson’s crimes.
The 28-year-old former England footballer was told on Wednesday he will receive a “substantial prison sentence” after he was found guilty of sexual activity with a besotted 15-year-old fan.
Sunderland sacked him after he pleaded guilty at the start of his trial at Bradford Crown Court to grooming and kissing her.
Manager Sam Allardyce told his weekly press conference that that had come as a “massive shock” to the club as bosses thought he would deny all charges.
In a statement after the trial, the club insisted they would have sacked him immediately had they known he was going to admit any of the charges.
But Detective Inspector Aelfwynn Sampson, of Durham Police, told BBC News that after the player was arrested a year ago, she told club bosses that sexual activity had taken place and that Johnson had messaged the girl.
Sunderland initially suspended the player for a fortnight, then allowed him back into the squad as the club fought relegation last year. The club insisted this was because Johnson said he was pleading not guilty.
The detective told the BBC: “At the centre of this we have a 15-year-old girl who was an avid Sunderland fan and a massive fan of Adam Johnson, she describes him as her idol, she’ll want to know why he was allowed back on the pitch.”
There have been calls for the bosses to answer why he was allowed to carry on playing and picking up £60,000 a week, despite the club allegedly being given all 834 WhatsApp messages exchanged between the footballer and the teenage girl back in May.
Clare Phillipson, director of charity Wearside Women in Need, said allowing Johnson to play sent out an “absolutely dreadful” message as it led people to think he was probably innocent.
During the trial the jury was told that, by May 4, chief executive Margaret Byrne met Johnson and his barrister and she had all the messages exchanged and transcripts from police interviews.
Ms Phillipson said: “If that’s true, then at that point they definitely should have suspended him because, in allowing him to continue to play, tens of thousands of fans and lots of other people thought ‘Oh well, the club is still letting him play - the case against him can’t be very good, he probably is innocent’.”
Julie Elliott, Labour MP for Sunderland Central, said “there are questions to be answered”.
The NSPCC said: “If Sunderland AFC had known that Johnson had kissed the girl, prior to his guilty plea, then he should have been suspended pending the police investigation.
“He was a role model and it would have sent out a strong message that this kind of behaviour must not be tolerated at any level.”
At his press conference, Allardyce said: “I am hugely disappointed on Adam Johnson and what has happened, but my sympathies don’t lie with him, they lie with the victim and the family.”
The Professional Footballers’ Association said Johnson’s actions were “extremely disappointing” and showed the organisation still had work to do in educating players.
The Football Association also criticised the behaviour of the former England player, saying: “Our thoughts are with the victim and her family as they look to rebuild their lives after this traumatic ordeal.”