By George P. Hassett

A five-unit Cross Street home suffered $400,000 in damages from a fire Tuesday morning that left nine adults homeless, according to fire officials.

The fire started on the second floor of 23-25 Cross St. at approximately 1:04 a.m., said Fire Chief Kevin Kelleher. He said investigators believe it was an accident but the probe is ongoing.

Firefighters were on the scene until 3:32 a.m. and entered the building to knock the flames down, he said. The fire reached the third floor. The building is currently uninhabitable – its residents cannot return, Kelleher said.

While Somerville firefighters fought the blaze, fire crews from Everett, Cambridge, Boston, Arlington and Medford covered their stations.