Sunderland were aware of Adam Johnson's sexual activity with a 15-year-old fan over a year ago, according to police.
Durham Police detective inspector Aelfwynn Sampson said she met with Sunderland chief executive Margaret Byrne on March 2 last year, when Johnson was arrested.
Johnson was initially suspended before being reinstated, but on Wednesday was found guilty of one charge of sexual activity with a child.
In her meeting with Byrne, Sampson said some of the details were made clear.
"They were given detail that he had met the girl and sexual activity had taken place," she told BBC News.
Sampson added: "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."
On Wednesday, Sunderland denied having any knowledge of Johnson's intention to plead guilty to sexual activity with a child, stating they would have terminated his contract immediately if informed of his plan to do so.
Johnson pleaded guilty to one count of sexual activity with a child and one of grooming at Bradford Crown Court on the opening day of his trial on February 10, denying two further charges of sexual activity with a girl under the age of 16.
The 28-year-old, whose contract with Sunderland was terminated following his guilty plea, was found guilty of one charge of sexual activity with a child on Wednesday, having earlier been cleared of another charge.
His lawyer has stated Johnson intends to appeal the verdict.