An early morning three-alarm fire in Riverdale yesterday gutted a three-story garden apartment building and injured four residents and one fire-fighter.

Prince George's County fire officials said the fire at the Jefferson Hall Apartments, located at 5414 Kenilworth Ter., was probably accidental, but they said their investigation was continuing.

Officials said the fire caused $500,000 damage, including extensive damage to the structure of the 12-apartment, brick building.

Officials said the fire probably started on a sofa in on of the apartments and that a resident pushed the smoldering sofa from his apartment into a hallway, but it became wedged in a doorway.

Injured in the blaze were James Beller, 21, of the Cheverly Fire Department, treated for second-degree facial burns; Zelda Rowley, 28, who suffered a sprained ankle; Leonard R. Rowley, 45, treated for minor injuries, and Rusty Rowley, 16 months, als treated for minor injuries. Fire officials did not identify a fifth person injured. All were released from Prince George's General Hospital in Cheverly.

One fire department source said the fire spread quickly because the L-shaped building was constructed without barriers designed to isolate or impede flames.

About 50 firefighters were called to extinguish the blaze.