HOUSTON – Houston fire crews are responding to a fire on Main Street at the corner of Walker. The fire was first reported at 7:59 a.m. and smoke has billowed out of the building for hours.

People are asked to avoid the area.

See live updates below:

1:30 p.m. UPDATE:

Executive Assistant Fire Chief Richard Mann said the fire was finally "under control" after it started at about 8 a.m. Thursday.

While officials don't yet know what exactly caused the fire or where the source was, they believe the fire itself is in the middle of the building and is traveling between floors in void spaces that are common in older buildings.

The building was built in the 1930s and so the construction codes are 90 years old, Mann said.

There were two Maydays called during the 5.5 hour response and during one mayday call, a firefighter was injured. He was taken to the hospital for treatment and is OK, Mann said. A mayday is a distress call that indicates a life-threatening situation.

Officials expect the firefighting effort will continue for several more hours and crews are being rotated to give them rest.

"Right now, I don't anticipate being out of here before dark," Mann said.

A church and a residential building next door to the Main Street Market were both evacuated due to the smoke but there isn't any damage caused to either structure because of the fire, Mann said.

12:10 p.m. UPDATE:

Houston fire crews have deployed a drone as they continue to battle the 3-alarm fire at the Main Street Market in downtown Houston several hours later.

About 150 fire fighters continue to search for the source of the fire and three crew members were rescued from the building earlier this morning.

HFD’s drone has been deployed at 3-11 building fire on the 900 block of Main St. Crews are still working on scene. Please avoid the area @FireChiefofHFD @SheldraBrigham @abbycortez pic.twitter.com/EG5BuDFvSV — Houston Fire Dept (@HoustonFire) October 17, 2019

11:40 a.m. UPDATE:

METRO Houston said service to the Red Line was suspended between the Downtown Transit Center and Burnett Transit Center stations due to the HFD response to the fire.

"Bus shuttles are operating in the affected area," the agency tweeted. "We'll provide an update once rail service is restored."

Officials from the fire fighter's union tweeted that they met with the injured firefighter and that he was stable. His family was notified.

11:25 a.m. UPDATE:

Three firefighters were rescued from the building, according to the Houston Professional Fire Fighters Association. Two are safe at the scene and one was taken to the hospital with injuries, the group wrote on social media.

An HFD official told KPRC the fire was actually at a 3-alarm response rather than 4 alarms as previously reported.

Many people trying to get to work at their downtown buildings remain stranded about 2.5 hours after the fire was initially detected.

10:45 a.m. UPDATE:

Watch the full update from Houston Fire Department Public Information Officer Abby Ramos below:

10:35 a.m. UPDATE:

Houston Fire Department Public Information Officer Abby Ramos told KPRC the source of the fire is yet unknown. One firefighter ran out of air during the response, and so he left the building and was found on the roof Ramos said. He was evaluated by not transported to the hospital.

The building has a main level and three floors above it. Crews have not yet pinpointed the source of the smoke and the whole building was evacuated.

GALLERY: See photos of HFD response to 3-alarm fire at downtown building

10:05 a.m. UPDATE:

The fire was upgraded to a 4-alarm response so more firefighters could find the source of the smoke. There are no flames visible from the street, according to KPRC reporter at the scene, Rose-Anne Aragon.

A firefighter was transported to a hospital from the scene. The extent of their injuries is unknown.

The fire response began as a 2-alarm fire. By 9:41 a.m., it was upgraded to three alarms so more units could respond and finally, it was upgraded to four alarms.

In a video tweeted by KPRC reporter Rose-Anne Aragon, smoke could be seen billowing from the building.

3 alarm fire. Firefighter was found on roof with trouble breathing but is conscious and was not transported. @kprc2 @HoustonFire Houston OEM on scene pic.twitter.com/yjBuGr5jG7 — KPRC2 Rose-Ann Aragon (@KPRC2RoseAnn) October 17, 2019

9 a.m. UPDATE:

KPRC 2's SKY2 is over the scene. See video below: