Up to 30 threatening letters containing white powder have been sent to banks across America but so far tests have shown them to be harmless, police said.
The letters began arriving on Monday and seemed to be related, officials said. The text has not been released.
No injuries were reported, although some bank staff were given a medical examination as a precaution.
The FBI - called in to help with the investigation - said that "even sending a hoax letter is a serious crime".
It said it was investigating "a series of letters sent to banks around the country".
"These threat letters contain a powder substance," the agency said in a statement. "At this point, field tests on the powder have been negative. Additional testing will be completed."
The US Postal Service has also launched an investigation.