Federal authorities are investigating at least 30 letters containing suspicious powder that have been sent to banks in seven cities.
The FBI said initial tests on the powder have come up negative for dangerous toxins. Additional tests are ongoing.
The letters all appear to be from the same source, and began showing up at the banks on Monday, the official said. The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly.
An FBI spokesman in Oklahoma says the letters sent there indicated the threat was "based on past actions of the bank."
The following statement was released by the FBI:
"The FBI, US Postal Inspectors and state and local authorities are investigating a series of letters sent to banks around the country. These threat letters contain a powdery substance. At this point, field tests on the powder have been negative. Additional testing will be completed. Even sending a hoax letter is a serious crime. If anyone has information, they are requested to contact the FBI, USPIS or local authorities. You can also go to www.fbi.gov to submit a tip. As this is an ongoing matter, we cannot discuss further details at this time."