More than a dozen other firefighters were injured.
“In 30 years, I’ve never seen a fire travel that fast, escalate that quickly, and create such havoc in such a short period of time,” Deputy Fire Chief Joe Finn told reporters.
He identified those killed as Lt. Edward Walsh, 43, and Firefighter Michael Kennedy, 33.
Walsh had worked with the Boston Fire Department for close to 10 yeas, while Kennedy had 6 1/2 years on the job. Walsh was married with three kids, all under the age of 10. Kennedy was a veteran of the U.S. Marine Corps.
“These two heroes ran into a burning building — got people out of the building,” said Boston Mayor Martin Walsh. “We lost two heroes here today.”
According to Finn, firefighters were able to rescue a number of people stuck on upper floors.
He said Walsh and Kennedy became trapped soon after entering the building. They were both later located in the basement, where the fire appears to have started.
Fueled by strong winds, flames quickly engulfed the four-story building.
At one point, there was an explosion and a number of firefighters were blown down stairs, Finn said.
“That fire … was blowing like a blowtorch out the front, from the rear to the front,” the deputy fire chief added.
In addition to those killed, 13 firefighters were injured. Some suffered burns, others broken bones.
On its Twitter page, Boston Emergency Medical Services said that it had transported 18 patients to area hospitals from the Beacon Street blaze.
The area is west of downtown Boston, near the Charles River. Video showed smoke rising over the city.
More than 150 firefighters and between 20 and 30 trucks responded to the incident, according to Boston Fire Department spokesman Steve MacDonald.
The cause of the blaze was not immediately clear. It’s under investigation.
Mark Bristol, an eyewitness, told CNN affiliate WCVB, that he saw people climbing down the building’s fire escape.
“I watched it go from pretty small to really, really big — like there was just smoke absolutely pouring out the front entrance,” Bristol said.