Three of those injured were hospitalized in serious condition. Injuries to the other people, including the four firefighters, were minor.
An emergency call about the fire at the five-story brick building, which has businesses on the first level and apartments above, came in around 12:45 p.m., the fire department said. The explosion and fire shattered windows on the first three floors.
When firefighters got to the scene, "we opened the front door and could see the fire in the back," fire Assistant Chief Robert Boyce said. It was unclear what caused the explosion on the first floor, he said, and there was minimal collapse inside the building but nothing structural.
Building resident Tszkan Cheung, who had been in his fourth-floor apartment eating lunch at the time, described what he heard as "boom, like a bombing, like an earthquake."
He made it out of the building on his own but saw firefighters carrying out a woman with a severely damaged leg.
Resident Jinjoo Yang, who lives next door, said, "I heard a big sound. It sounded like something big fell from the next floor. I felt the whole floor shaking."
She said gray smoke, smelling like gas, went into her apartment.
Boyce said one victim had to be taken out of a third-floor window.
The fire was under control shortly after 1:30 p.m. The cause of the fire hadn't been determined.