LYNN – A two-alarm fire caused extensive damage to a Salem Street duplex Monday night, displacing two families who were able to escape the flames.Firefighters responded to a report of flames in the rear of a two-family home at 20 Salem Street at 11:15 p.m., and called a second alarm just four minutes later as they arrived to find heavy flames and smoke billowing from the home.Everyone living in the home had made it out safely by the time firefighters arrived, and rescue crews were able to knock the flames down before they completely destroyed the home.”The home is not habitable at this time, there were some heavy flames in the rear of the first and second floor apartments,” said District Fire Chief Lee Oliver. “The men made a good, aggressive attack on it and were able to push it back and prevent it from spreading to the front part of the home. I believe that it will be repairable.”Oliver said fire crews, including the arson squad, are still investigating the cause of the blaze, but have focused their attention on the rear of the home.