INDIANAPOLIS – Ian Crocker’s chance to beat Michael Phelps was gone in a flash.

The anticipated renewal of their rivalry in the 100 butterfly Thursday was upstaged by an inadvertent strobe light that went off just before the start, causing Crocker to flinch and eventually be disqualified.

Fans still got to see the duel play out in the pool, and Phelps pulled away from Crocker in the final 25 meters to win in 51.39 seconds and break his own long-course record at the national championships. Crocker, whose time was quickly removed from the scoreboard and replaced by the dreaded disqualification symbol, said he swam a 51.6.

“It’s one of those things and I guess it’s a learning experience for myself, and, hopefully for U.S. swimming and people like that, too,” said Crocker, a former Cheverus of Portland star. “I’ve never had that to happen to myself and I’ve never known it to happen to anyone else.”

Jamie Fabos, a spokeswoman for USA Swimming, said officials were trying to determine whose flash went off. If they do, the offending news organization’s strobe will be revoked for the rest of the meet, Fabos said.

Crocker knew almost immediately the race was over before it actually started.

When fans in Indianapolis gasped in unison, Crocker figured that if the crowd had seen it, the officials surely had, too. They did.

Crocker’s coach also told him the flinch made him the last swimmer off the starting blocks, slowing the world-record holder.

Both Phelps and Crocker acknowledged there is usually a light that flashes, signaling the start of the race. To avoid getting confused, Phelps keeps his head down.

“I don’t go by the flash, I go by the sound,” Phelps said. “You could see it very clearly on the replay that the strobe went off.”

Crocker usually keeps his head down, too, since he’s been instructed to the look at the wall. This time, however, the flash apparently reflected off the water, distracting Crocker.

“I’m always attuned to react toward the flash,” he said. “That’s something that’s usually monitored very closely at the larger meets.”

But it didn’t bother Phelps at all.