Hazardous material teams responded to branches in Jamesburg, Monroe, North Brunswick, South Brunswick, Howell, Lakewood, Toms River and Watchung.
A law enforcement official says the letters all appear to be from the same source and were sent from Texas.
A city official in suburban Chicago, where two letters showed up at a Chase credit card processing center, says they were mailed from Amarillo.
The U.S. Postal Service and state and local officials are also investigating.
No injuries are reported.