REVERE – A Nahant Avenue apartment building is under investigation after its alarm system failed to alert emergency dispatch of a fire in the three-story building Monday.Firefighters were only alerted of the fire in 54 Nahant Ave. #2 when tenant William Seyum called 911 after returning home Monday just before 4 p.m. to find smoke seeping into his third-floor apartment from the unit below. Seyum said before calling he tried to get into the apartment but the door was locked.Revere Fire Deputy Chief Chris Bright said firefighters on scene knew where the fire was located in the building because of an alarm going off inside the apartment. Bright said once they forced entry into the apartment, they found the fire in the rear bedroom and put it out in about 20 minutes. No one was home in the apartment at the time.City Councilor John Corraggio watched firefighters take down the blaze as he stood on the sidewalk across the street. A former Revere Fire lieutenant, Coraggio said it was a good thing firefighters took care of it so quickly.?Especially for this type of neighborhood, with the buildings so close together,” said Coraggio. “A fire would be difficult in a neighborhood like this one.”Bright estimated the damage to be about $50,000 between the water damage to the first floor and the smoke damage to the upper floors. No one was injured, but tenants in the apartment with the fire and the apartment right above it are currently displaced, said Bright. He added that a bird and a cat were both rescued from the building, with another cat unaccounted for, which Bright suspected had run off.As firefighters were rolling up the hoses, Bright said fire prevention crews were on scene interviewing people in the building to determine the cause, and also to find out why the alarm system was broken.