PHILADELPHIA, Pennsylvania (CNN) — The explosion happened without warning around 4 a.m. in West Chester, Pennsylvania.

FBI agents were sitting in this car when a “flash-bang” grenade on one of the agents went off without warning.

FBI agent Donald Bain was sitting in his car in a parking lot with two other agents. He was armed and wore a Kevlar vest. He was also carrying a "flash-bang" grenade, a nonlethal weapon that emits a bright flash and deafening bang that’s used to shock and disorient criminal suspects or the enemy in combat situations.

The three agents — Bain, Thomas Scanzano and James Milligan — were waiting for developments on a kidnapping that had turned into a hostage stakeout.

That’s when, Bain says, the flash-bang grenade in his vest just blew up.

"The car is on fire," Bain recalled. "I was told later I was on fire. Smoke billowing in the car. It was obviously chaos."

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Scanzano remembers "it was like being in combat. There was smoke and fire in the vehicle, and I knew that we were in trouble."

An ambulance rushed the three agents to a nearby hospital.

"To me, it felt like someone just whacked me in the back with a baseball bat as hard as they could," said Bain, recalling the incident, which happened four years ago.

Bain suffered severe bruising, a concussion and burns to his neck and ears. All three agents said they have experienced hearing loss.

"There was smoke, and it was like a grenade going off in the car," Scanzano said.

The company that manufactured the flash-bang grenade that Bain used is Pyrotechnic Specialties Inc., also known as PSI, based in Byron, Georgia.

Earlier this year, PSI, its chief operating officer, David Karlson, and three other defendants were indicted for fraud, conspiracy and money laundering. According to the federal indictment, PSI had a $15 million contract to supply flash-bang grenades to the military before it supplied them to the FBI.

Watch soldier describe losing his hand from flash bang »

The indictment states the company knew its flash bangs were defective and even knew how to fix those flaws, which would have cost PSI $3.72 per unit.

But, according to the indictment, many of the defective grenades the military was using were relabeled and then sold to the FBI and other local law enforcement agencies.

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