REVERE – The Revere and state arson squads are investigating a fire at 60 Highland St. that caused at least $100,000 worth of damage Wednesday morning, according to Deputy Fire Chief Greg Menard.Crews arrived at about 10:15 a.m. and said the fire stretched from the ground to the roof of the rear side of the multi-family home.Menard says the fire began on a couch that had been placed up against the outside rear of the structure.”It’s possible the fire was lit,” Menard said.Arson investigators aren’t sure if young children may have accidentally set the fire and say the cause is still under investigation, Menard said.The deputy said two apartments suffered “substantial damage.”He added that one of the apartments appeared to be an illegal apartment in the basement of the house. The structure, owned by Vanessa Armano of the Grande Trust, only had four legal apartments, he said.The building inspector was being called in Tuesday to determine the legality of the apartment.The Red Cross was on scene to assist the approximately 12 to 15 people displaced, Menard said.Mutual aid was provided by the Boston, Winthrop and Chelsea fire departments.