TUSTIN – A fire inside a two-story office building quickly consumed much of 50,000-square-foot building, authorities said.

Nearly 120 firefighters were fighting the blaze, which continued to burn inside the two-story building for more than two hours. Three people were injured in the fire: two firefighters and a 62-year-old man inside an office where the fire began, officials said.

Officials received initial reports of a couch on fire inside one of the offices Tuesday morning, said Battalion Chief Kris Concepcion of the Orange County Fire Authority. Flames quickly spread through the front of the building, forcing firefighters to restrict their attack from the outside, Concepcion said.

“No one is inside,” Concepcion said while firefighters were battling the blaze. “It’s because of the size of the flames. We can’t send someone inside.”

Part of the roof of the building collapsed just minutes after firefighters arrived.

The fire was reported at 7:39 a.m. at 17291 Irvine Blvd. Firefighters from the Orange County Fire Authority, Santa Ana and the Orange Fire Department responded, said Capt. Greg McKeown of the OCFA.

The fire was reported by several people who were inside the building when the fire began, Concepcion said. The employees noticed fire and smoke before calling 911.

The blaze is believed to have started in a first-floor mental health services office. A 62-year-old man inside that office suffered minor burns to the face. He was treated at the scene.

Firefighters arrived minutes after the first call was received and immediately requested additional units, officials said. When firefighters arrived, they reported heavy smoke and flames coming from the front of the structure.

Flames appeared to have spread fast through the building, but the cause and origin of the fire is still unknown. The first fire truck reported seeing smoke from a mile away, Concepcion said.

By 8 a.m., officials asked for more units, resulting in about 80 firefighters, McKeown said.

Thirty minutes later, firefighters asked for additional trucks and firefighters.

One of the firefighters fell during the operation and suffered injuries to his face and legs, Concepcion said. A second firefighter suffered from possible heat exhaustion.

Both were taken to nearby hospitals to be treated, he said.

Harald Poth, who lives nearby, said the office buildings in the area are usually empty at about 8 a.m. When he heard sirens, the parking lot appeared to be empty, he said.

The building is home to several businesses that lease space, including legal firms, mental health services and real estate offices. The building is about 40 years old, Concepcion said.

At one point, firefighters had four strong streams of water aimed at the building from ladders, Concepcion said.

By 9:40 a.m., firefighters began to scale down their attack.

Officials began to release fire units at about 10 a.m., Concepcion said, but small flames continued to burn inside the building. The fire was declared under control before 10:30 a.m.

Contact the writer: shernandez@ocregister.com or 949-454-7361