CAMBRIDGE, Mass. (AP) _ An electrical explosion at a 17-story office building killed a utility worker Friday and sent dozens of people to hospitals with smoke inhalation, authorities said.

Two NStar utility workers were conducting maintenance in a basement vault supplying the building's electricity at the time of the blast around 11 a.m., Fire Chief Gerald Reardon said. Kevin Fidalgo, 28, died and Christopher Carey was treated for smoke inhalation.

Authorities were investigating the cause of the explosion at the Massachusetts Institute of Technology-owned building.

Most of the more than 30 people sent to hospitals tried to escape through stairwells but turned around after they were overcome by smoke, Reardon said. After smoke cleared, firefighters escorted them out.

Fire officials will investigate why smoke seeped into the building's stairwells, which are designed to prevent smoke from entering.