White Elephant Flea Market along the East Freeway in Channelview burned to the ground Friday morning.
Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives National Response Team were at the scene Sunday to investigate what happened.
Two dozen agents searched the remains along with Harris County fire marshal officials already on scene.
Authorities called the three-alarm fire suspicious because someone broke into two adjacent buildings, and three people were seen leaving the area when it started.
About 100 booths were damaged in the blaze. Investigators estimate the fire caused $750,000 worth of damage. No one was injured.
ATF agents were previously activated to investigate the Gallery Furniture fire in 2009 and church fires in Tyler and Temple in 2010.