A two-alarm fire tore through an office building at 67 Winthrop Street last night, destroying a Civil Liberties legal aid office and causing substantial damage to Herman and Lees, a graphics design corporation.

The first alarm was sent out at 5:20 p. m., and by 5:35, about 20 fire-fighting vehicles from Cambridge and Somerville had answered the call. By 6 p. m. flames had ripped through the ceiling of the building, and the temperature had dropped below freezing, adding new elements of danger to the task of fighting the fire.

At 6:15, though, the fire was under control, and the flames were killed although at that time "we were still chasing fire in the walls," Deputy Fire Chief Joseph Healey said after the blaze was quelled.

Mushroom

"It started in the back on the first floor, shot up through an air shaft and mushroomed out on the third floor," he said.

There were no serious injuries, although a few firemen were cut while fighting the fire. The cause of the fire was still undetermined when it was put out, although most of the speculation centered around the possibility of an electrical fire. Several firemen cited the fact that lights had been left on in the building and that the fire seemed to have started in the walls as possible evidence for an electrical failure which might have started the fire.