A human trading error at a major firm was the root cause of Thursday’s sudden, 9 percent selloff in U.S. stocks.

Multiple sources said a trader entered the letter “b”—as in “billion”—when he or she meant to type “m,” for “million,” shortly before 2:47 p.m. New York time.

U.S. stocks plunged suddenly, briefly by more than 9 percent, before pulling back to a near 3 percent drop, as investor worries mounted that Greece’s debt problems could spread.

Various sources are pointing to Citigroup as the firm in question.

Citigroup said it has no evidence of a bad trade but it is investigating the situation.

The New York Stock Exchange reported there were no computer glitches in its systems Thursday.

[via | thanks @kimparsell for the heads up too]