San Francisco police say that a candle was behind fire in the Richmond District this morning that burnt several buildings and caused multiple injuries.

According to tweets from the San Francisco Fire Department, the blaze began at around 1:46 a.m. at a three-story building at 453 23rd Avenue, which is between Anza Street and Geary Boulevard. Soon, the fire had spread to multiple adjoining three-story buildings, bearing the addresses 451, 455, and 457 23rd Avenue. A third alarm was declared at 2:02 a.m.

According to the SFFD, 31 fire units reported to the scene at the height of the blaze, with 96 firefighters fighting the flames.

Adding to the challenge for firefighters, CBS 5 reports, when responders entered the burning building, "they were confronted by hallways and stairs cluttered by hoarded possessions."

The SFFD says that two adult residents were injured in the blaze, and were transported to St. Francis Hospital for treatment of burns and smoke inhalation. According to the San Francisco Police Department, both were men, one aged 62, the other aged 75. As of publication time, they remain in critical condition. According to the Chron, a child was also hospitalized with minor injuries, and a firefighter was hurt during the blaze, but has been released from the hospital following treatment this morning.

A cat was also rescued from the flames, according to the SFFD. He or she appears to be fine.

By 3:15 a.m., the fire was under control, the SFFD says. ABC 7 reports that 25 residents were displaced in the fire, and are being supported by city services and the Red Cross.

The SFPD said this morning that the fire began when one of the hospitalized adult victim's "blanket caught on fire by a lit candle," but SFFD says the blaze remains under investigation -- and according to KRON 4's Will Tran, "Power was still on so investigators don't suspect it was candle."

One witness to the blaze concurs with that assessment:

As of Friday morning firefighters remained on scene ensure the fire does not reignite, and, presumably, to come to a final decision on the cause of the blaze.

Related: Candle Blamed For Deadly West Oakland Fire