The FBI said it was investigating "a series of letters sent to banks around the country."
"These threat letters contain a powder substance," the FBI said in a statement. "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."
A law enforcement official said the letters were mailed to Chase bank branches in or near Chicago, Cincinnati, Dallas, Denver, Newark, N.J., New York, Oklahoma City and Washington.
The letters all appear to be from the same source and were sent from South Texas, the official said. They began showing up at the banks on Monday, according to the official who spoke on condition of anonymity because he was not authorized to discuss the matter publicly.
The U.S. Postal Service and state and local officials are also investigating. Postal inspector JoJan Henderson says the letters appear to be related.
No injuries were reported. JP Morgan Chase & Co. spokeswoman Mary Jane Rogers says some employees, including a pregnant woman, were examined as a precaution.
Neither the local Postal Service nor the FBI has released the text of the letters or their origin.
Eight banks in the Denver area and eight in the Oklahoma City area received letters containing white powder, officials there said. All Denver branches reopened Tuesday.
In Oklahoma, the state Department of Health is conducting tests on the substance found in the notes, but Gary Johnson, an FBI spokesman there, said preliminary assessments done locally determined it to be harmless calcium.
Johnson said the Oklahoma letters indicated that the threat was "based on past actions of the bank" and that the letters implied that the opener was going to die.
Additionally, The Denver Post reported that one envelope that was opened by a machine Monday night spilled powder at a Chase credit card processing center in Elgin, Ill. Another letter was received there Tuesday, the newspaper reported. Employees there washed themselves off and did not report injuries.