Yesterday, just after 1200 hours a 23 box was transmitted alerting Engine Co.37 as the 3rd due engine to a reported apartment fire with people trapped. While responding, units were updated that someone had possibly set the apartment on fire and were still inside. Just prior to the arrival of the initial companies, PSC updated units that people had possibly jumped from the windows. The first due engine went on scene with smoke and fire showing from the apartment building… Engine Co.37 arrived and secured their own water supply and stretched the 400′ and a rack to complete the stretch. Once in the rear the engine officer gave the side charlie report of 4 stories in the rear and fire showing from division two. The engine gained entry and stretched to the floor above checking for extension.