Witnesses saw a flash of light from inside the vehicle, a red Jeep with a gray top, as it drove down a residential street in north Spokane, police said.
Donald Wilkes, 61, said the blast rattled his house and woke up everyone inside. He initially thought someone had crashed into his parked truck, and when he ran outside he found the street filled with smoke and the Jeep stopped just against his 6-foot-tall cedar fence.
"I looked around for something that got hit, but there was nothing," Wilkes said. "My son reached in to pull the keys out of the ignition and make sure he didn't go anywhere, and that's when we saw his hand was missing. It blew it right off at his wrist - they found part of it half a block away."
Wilkes' son, 30-year-old Nicholas, and another neighbor applied a tourniquet to the man's left arm. The man was stocky, estimated at about 28 to 30 years old, and coherent. But he wouldn't answer questions about what he had been doing, Wilkes said.
"All he did was look at my son and say, 'Oh God, oh God.' He looked like he was going to pass out," Wilkes said. "My main concern was why he was driving around my neighborhood at 1 in the morning with an explosive device."
The explosion blew out several of the Jeep's windows, knocked off its dashboard and splattered blood all over the driver's side door, he said.
Police and medics arrived within minutes and applied further tourniquets, then brought the man to a hospital, where staff credited the first aid with saving his life.
Officers cordoned off streets in the neighborhood and urged Wilkes' family to leave, but Wilkes, who has lived in the neighborhood for 30 years, said the bedrooms are on the other side of the house so they eventually just went back to bed.
A bomb squad used a robot to find additional explosives in the Jeep and remained on scene until about 7 a.m. defusing them, said Spokane Police spokeswoman Monique Cotton. The federal Bureau of Alcohol, Tobacco and Firearms took over after that and would ultimately determine whether criminal charges would be filed.
Cotton called it an isolated incident and said there was no further threat to the community.
Authorities eventually determined the explosion was caused by an M-1000 firecracker, Cotton said.
An M-1000 is a cylindrical firework, sometimes sold in boxes promising "maximum blast, super loud."