The Feb. 9 fire that resulted in the largest backdraft in U.S. history may have originated in a basement crawl space at the St. John Evangelical Lutheran Church, 327 S. Main St. in Sycamore, Sycamore Fire Chief Bill Riddle said.

The crawl space held the organ blower system that caught fire during church service the day before and was extinguished quickly. Fire crews then went through the area with thermal-imaging cameras and deemed the area to be safe, Riddle said.

No one noticed anything unusual before the fire.

“Fires can be a funny business … there can be a small ember hidden,” Riddle said.

Sometime between 4:15 p.m. when the last person reported leaving and the 911 call at 4:59 p.m., the fire moved through the church and consumed enough oxygen to create a backdraft situation, Riddle said.

Shortly after 5 p.m., fire crews responded to the 66-year-old church and found a plume of smoke rising from the church basement. The church, with its stained-glass windows and stone exterior, was retaining enough heat to create a disaster, Riddle said.

When firefighters arrived, the fire had consumed much of the oxygen in the church, reducing flames from the fire.

Firefighters assessing the fire made a life-saving decision to not send crews in immediately, Riddle said.

When firefighters introduced oxygen to the fire, the backdraft blew the roof off the church. The explosion sent seven firefighters to the ground.

Firefighter Brad Belanger was injured by the explosion after he had removed his glove to determine how hot a rear door was and whether it was safe to enter, department chaplain Donald Phelps said. Belanger was treated for first- and second- degree burns to his hand and was released that day.

Firefighter Bill Reynolds is expected to be released from OSF St. Anthony Medical Center in Rockford, where he has been undergoing treatment and surgery for leg injuries he suffered when debris from the explosion fell on his leg. He will enter a rehabilitation program upon release, Riddle said.

Fire investigators have wrapped up the investigation at the church and turned the property over to insurance carriers to investigate the estimated $4 million in damages.

The Bureau of Alcohol, Tobacco, Firearms and Explosives concluded that the backdraft at St. John was the largest documented backdraft in United States history.

A complete report is expected to be released Monday.