WASHINGTON (AFP) -- The FBI said it was investigating Friday a case in which more than 50 letters, most containing a white powder, were mailed to U.S. financial institutions threatening ""payback"" for the financial crisis.

The Federal Bureau of Investigation said in a statement on its website that the letters postmarked in Amarillo, Texas began arriving Monday ""causing a massive response and ensuing multi-agency investigation led by the FBI and the U.S. Postal Inspection
One letter, part of which was posted on the FBI's website, warned that no one could ""steal tens of thousands of people's money and not expect repercussions (sic). It's payback time. What you just breathed in will kill you within 10 days. Thank ... and the FDIC for your demise.""
The FBI said tests so far had shown the powder was harmless. ""Most of the letters contained some sort of powdery substance. All field tests to date have turned up negative -- the powder appears harmless,"" the FBI said, adding that ""additional testing is taking place at regional laboratories.""
Letters have been sent to at least 11 states, including Arizona, California, Colorado, Georgia, Illinois, New Jersey, New York, Ohio, Oklahoma, Texas, and Virginia, the FBI said.
And recipients have included Chase Bank; the Federal Deposit Insurance Corporation, or FDIC; and the U.S. Office of Thrift Supervision, which regulates all federal and many state savings institutions.
A global credit crisis has been widening since a U.S. crisis of confidence over sub-prime mortgage securities helped lead to the collapse of a major Wall Street player, investment firm Lehman Brothers.
Lehman sought bankruptcy protection on September 15, after frantic talks failed to find a buyer for the 600-billion-dollar Wall Street giant and the U.S. government decided not to step in and save it.
The collapse of Lehman Brothers, one of Wall Street's most established banks, sparked turmoil on financial markets across the world.
Governments around the world have unveiled packages over the last month totaling more than three trillion dollars, including loan guarantees and cash injections, to restore confidence to banks and reverse a drop in lending.
U.S. authorities have been on the alert for powdery substances sent by U.S. mail since 2001, when anthrax in letters killed five people.
In August microbiologist Bruce Ivins, 62, killed himself as prosecutors were preparing to charge him in the 2001 attacks-by-mail that killed five people and left 17 others sick.