The blaze struck an apartment complex that was under construction in Waltham before dawn Sunday morning.Scott Eisen for The Boston GlobeFirefighters from across the Boston area responded.Scott Eisen for The Boston GlobeNo injuries were reported, although five buildings collapsed.Scott Eisen for The Boston Globe/Globe FreelanceThe fire’s cause wasn’t immediately known.Scott Eisen for The Boston GlobeThe fire struck an apartment complex that was under construction.Scott Eisen for The Boston GlobeBuildings collapsed during the blaze.Scott Eisen for The Boston Globe