Federal authorities say more than 30 threat letters were sent to JP Morgan Chase banks around the country, all postmarked last Saturday, October 18, in Amarillo, Texas.
An FBI spokesperson said the letters all contained a powder substance that appears to be harmless. Additional testing is underway.
The letters went to Chase bank facilities in at least seven cities, according to the FBI, and began showing up yesterday.
"Even sending a hoax letter is a serious crime," said the spokesman, Richard Kolko.
Law enforcement authorities say they will be able to determine in the next day the precise post office or postal box used to mail the letters.
The FBI spokesman said the letters indicated the threat was based "on past actions of the bank."
The mailings do not have any apparent link to the upcoming election or terrorism. Federal law enforcement officials believe the sender may be motivated by a personal grudge against the bank.
A spokesperson for JP Morgan Chase said bank officials are cooperating with the FBI, postal inspection services and local police "to handle the matter quickly."
The letters forced some of the Chase branches to shut down. In several of the cases, bank employees who opened the letters were decontaminated and treated for exposure to the white powder, but none of the tellers had any symptoms.
Branches in Colorado and Oklahoma have returned to business as usual today, and in New Jersey branches that were temporarily closed have reopened or soon will reopen at the guidance of the Postal Inspection Service.
A New Jersey branch received a threatening letter today, while branches in Oklahoma and Colorado received letters yesterday.
Susan Caraher and Mark Crudele contributed to this report.