At least 30 letters containing suspicious powder have been mailed to Chase banks in nine cities.
Investigators zeroed in on possible suspects in Texas after letters with a suspicious powder were received at some Chase banks.
Banks in at least nine cities -- including Dallas -- received the letters carrying an Amarillo postmark.
The FBI says initial tests on the powder showed it to be harmless calcium.
Investigators say most of the 30 or so letters contain a powder substance with a threatening communication.
An FBI spokesman says the threats were "based on past actions of the bank" and that the letters implied that the opener was going to die.
No injuries have been reported.