A 6-alarm fire that investigators said was deliberately set injured more than a dozen people and prompted dramatic rescues early Monday, fire and police officials said. A man who said he started the fire was arrested, police said. The blaze damaged or destroyed more than two dozen apartments in the large, U-shaped building on Wardman Road in the city\'s Roxbury neighborhood. Residents who escaped the blaze said they heard an explosion in the building at about 1 a.m. A short time later, fire department spokesman Steve MacDonald told the Associated Press a man showed up at a hospital and said he had started the fire. \"He walked into Boston Medical Center with burns and admitted certain things to investigators, so he was placed under arrest for arson of a dwelling,\" MacDonald said. The man\'s name was not immediately released. It was not clear if he lived in the building or was visiting, MacDonald said, and no motive was known. Firefighters rescued at least 15 people from the building, including a young child who was dropped from a third-floor window, MacDonald said. Third-floor residents Herbert and Judith Lamb lived in the third-floor apartment and said they heard a \"loud explosion,\" followed by smoke quickly filling the building. The Lambs broke a window in the apartment and dropped their 6-year-old grandson into the arms of waiting Boston firefighters. \"I had to drop him out the window,\" Judith Lamb told The Boston Globe (http://bo.st/qBIQcQ ). \"The firemen caught him.\" Boston Fire Lieutenant Glenn McGillivray told the newspaper he was the one who caught the child. \"I knew she wouldn\'t be able to hold him until we got help, so they dropped him, and I caught him in my arms from the third floor,\" McGillivray said. \"Then we got a ladder over to get the woman out.\" \"Thank God I caught him. I\'ve never had to do that before, and I hope I never have to do it again,\" McGillivray said. Ten residents of the building were taken to hospitals, most suffering from smoke inhalation. Two firefighters and a police officer were also treated for what were believed to be minor injuries, MacDonald said. Firefighters said later Monday morning that had accounted for all residents, though MacDonald cautioned that crews had not yet been able to enter the building. About 160 firefighters were called to the scene and they were still pouring water on the building several hours after the blaze started. A temporary shelter was set up in the city for residents displaced by the blaze. \"There appears to have been some kind of explosion,\" MacDonald said, adding that in one corner of the building the windows and frame had been blown out and bricks were littering the street. He could not immediately confirm initial reports that the fire may have been started after a gas line to a stove was cut.