LIVERMORE (KRON/BCN) — A fire that burned a Livermore apartment building and displaced 21 people early Sunday morning has been deemed accidental, fire officials said.

At 3:01 a.m., firefighters responded to a report of a fire at a triplex on Chestnut Street, according to Livermore-Pleasanton Fire Department officials.

Upon arrival, firefighters saw flames coming from the front windows and door of a second-floor unit, fire officials said.

A family of four was able to escape the fire from their second-floor apartment, according to fire officials. Firefighters quickly contained the fire to the apartment.

The unit sustained major damage, Fire Battalion Chief Paul Legasa said.

Fire officials determined the blaze was not intentionally set, but an official cause has not been determined as of this morning, according to Legasa.

The fire displaced all 21 occupants of the triplex. The American Red Cross is providing relocation assistance to the occupants, fire officials said.