Federal authorities investigating a building fire that injured six Detroit firefighters say the blaze on the city’s east side is a case of arson.

Four firefighters remained hospitalized Monday.

READ MORE: Michigan Matters: Two Stars With Motor City Ties Lighting Up Entertainment World

The Bureau of Alcohol, Tobacco, Firearms and Explosives said at a news conference Monday afternoon that the cause of Friday’s blaze along East Jefferson near Dickerson was arson.

The initial fire at the two-story commercial building was extinguished after firefighters arrived at 5 a.m. Friday. The blaze was rekindled more than two hours later.

ATF Special-Agent-in-Charge Tom Brandon said investigators know the fire started in the Metro PCS part of the building, and although they know what was used to start the fire, he wouldn’t say.

READ MORE: 23rd Annual 'A Home For The Holidays At The Grove' Comes To CBS On Sunday, December 5th

“We do know, but right now it turns into a criminal investigation because accidental was determined not to be the origin of this fire. and now it’s an active criminal investigation so we’ll withhold any of the evidence so we hopefully can get the public’s help,” Brandon said.

Injuries to firefighters ranged from a strained back to a spinal cord injury when the building collapsed.

A 15-thousand dollar reward is being asked for information leading to the arrest of the arsonist. Anyone with information can call 1-888-ATF-FIRE.

MORE NEWS: MDHHS Updates COVID-19 Quarantine Guidance For Michigan Schools

© MMX WWJ Radio, All Rights Reserved. The Associated Press contributed to his report.