They finally admitted it after the Texas Rangers found proof that the FBI fired pyrotechnic flares. But the FBI was adamant that the pyrotechnic did not start the fire.
The ex-Senator doing the investigative report chose to suggest "no punishment" for the FBI agent that had lied (to Congress, as I recall) about use of the flares.
This info is in FR archives.
Clarification -- the FBI admitted using pyrotechnic flares after being caught by the Texas Rangers --but did not admit using incendiary tear gas .