A three-alarm fire that began just after midnight in a triple-decker at 58 Marlboro St. destroyed the century-old structure leaving the owner and two sets of tenants homeless, according to Belmont Fire Chief David Frizzell.

The nine residents of the building escaped the blaze uninjured.

The cause of the fire is unknown at this time and is under investigation, said Frizzell.

A 911 call came into BFD headquarters at 12:30 a.m., Wednesday, May 21, for a fire at the address near the Cambridge town line and Belmont Cemetery, Frizzell told the Belmontonian at the scene.

The blaze quickly turned into a inferno in the rear of the 3,850 square-foot building built in 1900 with heat so hot it melted the vinyl-siding of the house with the adjoining backyard and started fires in the adjacent structures.

At the height of the incident, seven engines and three ladder trucks fought the fire that was finally under control at 2:45 a.m.

“And we are still chasing hot spots now,” said Frizzell at 5:45 a.m.

The structure, in which the rear section has essentially burned away, “is a complete loss. There’s nothing that can be salvaged,” he noted.

Mutual aid in the form of fire apparatus from Cambridge and Watertown helped fight the blaze while Newton, Winchester and Somerville fire departments covered Belmont during the fire.

In addition, five private vehicles were destroyed in the flames.

“This was a pretty big fire,” said Frizzell who was still at the site this morning.