The agency has declined to release the man's name. Agents were waiting for the man to be medically cleared so they could interview him, ATF spokeswoman Cheryl Bishop said.
The explosion of an M-1000 firework occurred early Saturday morning inside a moving vehicle in a north Spokane neighborhood. Witnesses saw a flash of light from inside a red Jeep with a gray top as it drove down a residential street, police have said.
Several of the Jeep's windows were blown out, its dashboard was knocked off and blood was splattered all over the driver's side door, witnesses told police. People at the scene applied a tourniquet to the left arm of the man, who they described as stocky and estimated to be between 28 and 30 years old.
Police and medics arrived within minutes and applied additional tourniquets, then brought the man to a hospital, where staff credited the first aid with saving his life. A bomb squad used a robot to find additional explosives in the Jeep, Spokane Police spokeswoman Monique Cotton said.
The ATF has taken over investigation of the case.
"We are doing interviews with his known friends and associates," Bishop said on Monday. "We are trying to determine where these devices came from, why he had them and what happened."
Authorities have determined the explosion was caused by an M-1000. The firework is classified as "an illegal explosive device" for people not trained to use them, Bishop said.
The ATF can present evidence to the U.S. Attorney for Eastern Washington, who will ultimately decide if charges will be filed, Bishop said.
Cotton has called the explosion an isolated incident and said there was no further threat to the community.