LYNN – A fire in the rear of a multi-family apartment house at 70 Walnut St. gutted the building and left 15 people homeless Tuesday, according to fire officials.The fire also sent five people to the hospital suffering from smoke inhalation.It was the second time in five years the Walnut Street building caught fire, according to Fire Chief Dennis Carmody.Police rerouted traffic along busy Walnut Street near the intersection of Linwood Road as firefighters snaked hoses along the road and sidewalk, ensuring a steady water supply.”Right now we don’t know what caused the fire,” Carmody said at the scene. “We had two adults and three children taken to the hospital where they will be treated for smoke inhalation.”Most of the displaced occupants were kept warm inside ambulances at the scene while firefighters doused the flames and later checked for hot spots hidden inside walls and ceilings.Fire Lt. David Legere, head of the department’s arson squad, interviewed residents after they were checked by emergency medical teams.The blue vinyl-sided, three-story structure appeared unscathed from the front, but the rear was charred black, the porches weakened by heat and fire.Two residents recalled smelling smoke, but saw no flames and did not leave the building until the first fire companies arrived. By then, a black plume of smoke could be seen from blocks away, rising over the neighborhood at the base of Pine Hill.It was the second fire in Lynn in less than three hours. Firefighters stopped an apartment fire on Wilson Terrace shortly before 10 a.m. (See Page A3.)Red Cross representatives were scheduled to assist the homeless with finding temporary quarters at area motels.The fire alarm sounded a call for mutual aid, bringing a Swampscott fire engine to reinforce the Lynn crews.