Several buildings near the bank have been cordoned off, and about a dozen police cars, medical personnel, and the FBI are at the scene.
"The focus is on the safety of everyone involved," said a spokesperson for the bank.
"This is an active crime scene. If you are in the area, please leave," Police Chief Eric Celedon said in a video on Instagram.
Some of the hostages have reportedly managed to escape; it is unclear how many remain inside the bank. Negotiators have established contact with the man by phone.
The motive for the incident is unknown.
