A large blaze engulfed a two-and-a-half story residential building on Mount Vernon Street in East Somerville Tuesday morning.

"This thing was fully evolved when we arrived," said Somerville Fire Department Deputy Chief Jim Lucia of the blaze, which struck 81 Mount Vernon St. Rear.

Photos taken by a neighbor, whose home is mere feet from the burned-down structure, show high flames and thick black smoke emanating from the back of the building.

Lucia said the plume of "heavy black smoke" was visible from the McGrath Highway.

It was a two alarm fire, Lucia said. All fire units from Somerville responded to the incident, in addition to a unit from Cambridge and a unit from Boston.

No one was injured, the deputy fire chief said, but at six residents have been displaced and sought assistance from the American Red Cross.

One resident named Ryan, who did not want to give his last name, was visibly upset by the incident. He was wearing sweatpants and no shirt, having lost all his clothes in the fire.

Megan Zhang, 25, who lives at 81 Mount Vernon St., located just in front of the structure that burned, said her roommate smelled smoke and soon firefighters "came and knocked on our door and said, 'Everyone out.'"

The roommate, Natalia Miao, 23, said, "We didn't realize how heavy the fire was." It was Miao who took some photos of the flames.

As soon as they realized there was a fire, "about two minutes later [firefighters] came. So it was really fast." The two women did not know if they would be able to spend the night in their apartment due to some smoke damage.

No cause of the fire is not yet known, said Lucia. The Somerville fire investigator is investigating.