BARCELONA, Spain >> Matt Grevers scratched his head, wondering what went wrong.

Nathan Adrian and Ryan Lochte stared at the scoreboard, their jaws open in disbelief. Kevin Cordes stood on the deck with his hands on his head.

“That’s like a punch in the gut right there,” U.S. men’s head coach Bob Bowman said.

Moments earlier, all four swimmers were embracing each other, again celebrating that surest of gold medals in the pool — the men’s 4×100 medley relay.

It’s a race the U.S. has never lost, except for disqualification.

Since the race’s inception at the 1976 Montreal Games, the Americans’ only defeat at a worlds or Olympics came at the 2007 worlds in Melbourne, Australia, when Ian Crocker dove too soon for the butterfly leg in morning heats.

This time, the disqualification came during Sunday’s final before a crowd of 10,000 screaming fans inside the sweaty Palau Sant Jordi. And after Adrian touched the wall first in the anchor leg by nearly 1.5 seconds — an eternity in swimming.

Cordes, a 19-year-old competing at his first worlds, dove for his breaststroke leg 0.04 seconds before Grevers touched the wall to complete his backstroke leg.

The rules allow a leeway of 0.03, so it was 0.01 over the limit — the smallest margin possible.

“It’s as much my fault or more than Kevin’s,” Grevers said. “The guy coming in is usually the one responsible. I kind of slipped off my start and got a little more tired than I wanted to at the end.

“I probably had a slower rate than I was anticipating — obviously than he was anticipating,” Grevers added. “I got to learn to finish better and he’s probably got to learn to time a little better. I have a feeling Kevin’s going to be a little safe for the next couple years.”

When “DQ” was placed next to United States on the scoreboard, France moved up to take the gold, while the silver went to Australia and Japan snatched the bronze.

Afterward, Cordes was escorted past reporters without saying a word, while his teammates addressed the failure.

“A relay disqualification is not a particular individual’s fault,” Adrian said. “It’s Team USA’s fault and it falls on all of our shoulders.

“If us four ever step up again we’re never going to have a disqualification, that’s for sure,” Adrian added. “It will really motivate him. I don’t doubt if in the next couple years we’re going to have the fastest breaststroker in the world swimming for Team USA. This could be a catalyst for that.”

The entire meet has been difficult for Cordes.

After winning the 100 and 200 breast at the U.S. trials, his best result here was seventh in the 100. He failed to advance from morning heats in the 50 and just missed qualifying for the final in the 200, finishing ninth and missing the cut by 0.02.

“It’s a big step up to come here,” Bowman said. “This is a great learning experience, because DQ-ing a relay in the first world championships of the quad is one thing, doing it in an Olympic prep competition would be 10 times worse, right? So it’s a good learning experience and they have to go back and rethink how they’re going to react to things in this environment and do better.”

Until the DQ, the biggest concern for the Americans was how to replace the retired Michael Phelps, who swam the butterfly leg for the last decade.

Ryan Lochte took over that job and did it so well he put the U.S. back ahead after Cordes touched second. Lochte wouldn’t talk about his performance, though. His focus was the team.

“You take 0.04 off our time and we’re still a second in front of everyone,” he said. “We have all the speed and everything is right there. We just got a little carried away, and I guess things happen. But more fuel for the fire. We’re going to come back next year stronger and better.”