Letters containing suspicious white powder that were delivered to more than 50 bank branches and a prominent newspaper headquarters in the United States carried a message "it's payback time," the Federal Bureau of Investigation said yesterday.

"Steal tens of thousands of people's money and not expect repercussions. It's payback time. What you just breathed in will kill you within 10 days," said a letter released by the FBI.

However, the letters, all from Amarillo, Texas, did not contain the same wording, the bureau added.

Since Monday, branches of Chase Bank, the Federal Deposit Insurance Corp. and the U.S. Office of Thrift Supervision at 11 states and the District of Columbia as well as the New York Times headquarters have received such letters with white powders which were, however, confirmed as harmless calcium after preliminary tests.

Despite a hoax, the FBI said that sending threat letters is still considered "a serious crime" and the U.S. Postal Inspection Service has offered a reward of 100,000 dollars for information leading to the arrest and conviction of people responsible.