The FBI said today that it had investigated over 900 threats made with envelopes containing white powder probably meant to look like anthrax.

"Interesting enough, over the past two-years, the FBI has responded to over 900 of these threat letters," said FBI spokesman Rich Kolko. 

"And when I say they've responded to the threat letters, that means there has to be a threat letter with the powder. " Kolko said in a podcast posted online. 

"We've all heard of those innocent ones where someone spills some sugar on the table and someone calls the police. I'm not even counting those. But that means this has been a large problem across the country for the last few years."

On Thursday the FBI appealed to the public for help with finding the perpetrator who has been sending banks, financial institutions and federal agencies threat letters and powder-stuffed envelopes.

The New York Times also received a threatening letter with powder, but it was deemed not connected to envelopes directed at institutions involved with the mortgage crisis. 

"If you recognize the writing style or if something makes you think it could be tied to someone you are familiar with, we want you to pick up the phone, call the FBI; go to fbi.gov where you can submit an anonymous tip; call the postal inspectors; local police; whatever it takes," Kolko said Friday.

"Try and get that information to us. And very importantly, just on Wednesday the postal inspectors offered a $100,000 reward."