The key witness in one of the most significant white-collar crime cases in US history has spoken out for the first time.

Alayne Fleischmann, whose testimony was used by US prosecutors to negotiate a $US9 billion ($10.4 billion) settlement with her former employee JPMorgan, said her attempts to stop the bank's sale of dodgy loans were repeatedly silenced.

JPMorgan: forced to pay a $US9 billion settlement. Credit:Reuters

In an interview with Rolling Stone, The Cornell Law grad said she witnessed "massive criminal securities fraud" in the bank's mortgage sales in the months leading-up to the financial crisis.

"It was like watching an old lady get mugged on the street," she says. "I thought, 'I can't sit by any longer.'"