An explosive fire left 17 people homeless in Worcester Saturday, while firefighters escaped serious injury after a backdraft blew out a window on the fourth floor of the Lafayette Street building, showering glass and debris 30 feet away.

The 3 alarm fire heavily damaged the four-story apartment building at 46 Lafayette St., which broke out around 1 a.m.

As firefighters were climbing a stairway to the fourth floor, a backdraft explosion blew out windows on the upper floor, Deputy Fire Chief Geoffrey Gardell told The Telegram.

The firefighters were knocked down the stairs by the force of the blast, but none was seriously hurt, according Gardell.

The cause of the fire is under investigation.