CHULA VISTA, Calif. -- A fire of unknown origin displaced a family of four from their South Bay apartment late Tuesday.

The blaze began spreading shortly after 5 p.m. in a laundry room in the second-story residence in the 700 block of Caminito Obispo in Chula Vista, Deputy Fire Chief Jim Garcia said.

Two adults and two young children inside the apartment at the time -- a couple and their grandchildren, whom they were baby-sitting -- escaped unharmed, Garcia said.

The youngsters' father, however, arrived shortly after the fire erupted and suffered minor injuries to his hand when he used his fist to break glass in a fire-extinguisher box at the residential complex. Medics treated him at the scene.

It took firefighters about 15 minutes to extinguish the flames, according to Garcia.

The American Red Cross was called in to help the family arrange for interim shelter.

The cause of the blaze was under investigation.