At the first presidential debate, Hillary Clinton accused Donald Trump of "hiding something" by refusing to release his tax returns.

Trump repeated, after being asked about it by Lester Holt, that he cannot release his returns until an IRS audit is over. 

Clinton said presidential candidates have been releasing their returns for 40 years. 

She wondered whether Trump isn't as rich as he says, does not contribute as much to charity as he says, or possibly owes money to Wall Street and foreign banks. 

"It must be something really important, even terrible that he's trying to hide," said Clinton.

Watch the clip above.