INDIANAPOLIS — Ian Crocker spent months training for another chance to beat Michael Phelps in the 100 butterfly.

His hopes vanished before the race even started.

The rivalry between two of America’s best swimmers at the national championships ended when a strobe flash inadvertently went off just before the race started.

Crocker reacted with a flinch, and as the Indianapolis fans gasped, the world-record holder already knew his fate — disqualification.

“I heard the crowd when it happened, and I was pretty sure if they had seen the flinch, the officials had seen it, too,” Crocker said. “That’s their job.”

But the fans still got to watch the highly anticipated race, and Phelps won it anyway, pulling away in the final 25 meters with a time of 51.39 seconds to break his own long-course record at the national championships.

Crocker, whose time was quickly scrubbed from the scoreboard and replaced by the dreaded disqualification symbol, said he swam a 51.6.

The loss was tough enough, but the distracting light was even more disconcerting to Crocker.

“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,” Crocker said. “I’ve never had that 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.

Both Phelps and Crocker acknowledged there is usually a light flash, 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 and Crocker reacted.

“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.

The two-time Olympian, who set three personal world records and two more in relays at the world championships this spring, called it his best finishing lap ever in the 100 fly.

Phelps showed the swimming world something else Thursday: He’s tuning up for next year’s Olympics.

The victory in the 100 fly came 32 minutes after he finished third in the 400 freestyle.

Peter Vanderkaay, one of Phelps’ teammates with Club Wolverine, pulled away in the final 100 meters, winning in 3:45.55 — his personal best. Larsen Jensen was second in 3:47.08, and the hard-charging Phelps, was next at 3:47.13.

Phelps’ coach, Bob Bowman, said he was really using the event to learn how to conserve energy for next year’s Olympics when he’s expected to challenge Mark Spitz’s record of seven gold medals. Phelps pulled out of the 800 free relay, the last event of the night.

“That’s why we do this, and that’s what he did so well in Melbourne,” Bowman said. “He’s getting better and better at it.”

The Phelps-Crocker matchup was supposed to be the showcase event at this meet because it’s one of the few in which Phelps has consistently been challenged.

Crocker beat Phelps in world-record time at the 2003 world championships. At the 2004 Olympics, Phelps outtouched Crocker at the finish to take home the gold.

The next year, at the world championships, Crocker again beat Phelps in world-record time (50.40), only to have Phelps outtouch him again at this year’s world championships. Crocker said they may not face each other again until spring nationals.

Kate Ziegler, a 19-year-old, also was impressive.

She held off Katie Hoff, an Olympian in 2004, to win the women’s 400 free in 4:04.24 — a U.S. Open record. Hoff was second in 4:04.60.

It could have been even better as the announcer urged fans to cheer louder as they neared an American record pace. Janet Evans’ mark of 4:03.85, at the Seoul Olympics, is the longest standing American record for men or women.

“I just tried to get to the wall as fast as I could because Katie’s such a fierce competitor, and I knew she would push me the whole way,” Ziegler said. “I’m really, really pleased with how the race went.”

North Bend graduate Kim Jasmer finished 24th in the 400 freestyle with a time of 4:22.78. She led the C Final — for the qualifiers finishing between 17th and 24th in preliminaries — midway through the race before fading in the final half. In the morning, Jasmer was timed in 4:17.15 in qualifying.

Rachel Komisarz held on to beat Dana Vollmer, her Olympic teammate three years ago, to win the women’s 100 fly in 58.04. Vollmer touched in 58.48, and Christine Magnuson was third in 58.76.