Update — March 26, 10 p.m. ET: Two firefighters were killed in the blaze, confirmed Mayor Martin J. Walsh.
The two men killed were both from unit Engine 33 and were identified as Lt. Edward Walsh, 43, and firefighter Michael Kennedy, 33.
"Today's a sad day for the city of Boston," Walsh said. "We lost two heroes ... these two heroes ran into a burning building and got people out of the burning building."
The governor also tweeted his condolences:
My heart and my condolences go out to the families of the firefighters lost in the line of duty today...(1/3)
as well as to the entire Boston Fire Department. This terrible tragedy reinforces how we must be grateful every single day...(2/3)
for the brave men and women who put themselves in danger day in and day out to keep us safe. (3/3)
At least 16 other people have been injured in a nine-alarm fire in Boston's Back Bay neighborhood.
Among the injured were 13 firefighters in addition to the two men who have already died from the blaze at the 298 Beacon St. brownstone.
A law enforcement official told the Boston Globe that firefighters battling the blaze briefly found themselves trapped in the basement.
Lt. Edward J. Walsh was from West Roxbury and a father of three. Firefighter Michael R. Kennedy was from Hyde Park and was a Marine veteran.
Fallen @BostonFire heroes Ed Walsh and Michael Kennedy #wcvb
Lt. Walsh comes from a family of firefighters.
Lt. Walsh's late father was a Lieutenant as was his late Uncle Bill. His cousin Tom is currently a Captain, all on Watertown Fire
Multiple outlets reported that Boston police declared the fire a crime scene.
Police said the building, located on one of the most desirable blocks in the city steps from the Charles River, is a four-story brownstone, and added that the fire extended up through the building from the basement.
Watch footage from the fire:
View this video on YouTube
Check back for updates.
Correction: The governor of Massachusetts is Deval Patrick. An earlier version of this story misstated his role.