Denver fire officials say they now know what did not cause a deadly jobsite fire in early March that took the lives of two construction workers. After an extensive investigation that involved more than 200 interviews, officials have cleared most of the trades working at the five-story apartment building at the time of the fire.

Those not at fault include drywallers, plumbers, electricians, welders, framing crews and insulation installers. Also ruled out were temporary power units and electrical cords used on site. But Denver Fire Dept. spokesman Greg Pixley said in a June 27 statement that officials still don’t know exactly what caused the massive blaze, which trapped the two men inside the wood-frame structure. For now, the cause is officially listed as “underdetermined … and because of ongoing interviews and investigation, this case will remain open and under investigation,” Pixley said. The fire damaged seven nearby buildings and destroyed more than 40 cars parked near the site.