The crackdown on steroid use at the Pan American Games reached U.S. athletes today when weightlifter Jeff Michels was stripped of his three gold medals and 13 members of the men's track and field team returned home, all but one apparently because of the sophisticated testing methods here.

The 13 athletes, most of them entered in the weight events, left this morning a few hours before competition was to begin.

The U.S. Olympic Committee indicated at a press conference that most of them departed in response to the seven positive tests of weightlifters announced by the Pan American sports organization Monday.

The crackdown on steroid use claimed four more weightlifting victims today, including heavyweight Michels, who won three gold medals Friday and outperformed the super heavyweight winner.

Ordered to return medals, besides Michels, were Jose Adarmes Paez of Venezuela, winner of two bronze; Enrique Montiel of Nicaragua, two bronze, and Jacques Oliger of Chile, three silver.

Evie Dennis, chief of mission of the U.S. delegation here, said she had informed the coaches and managers of the U.S. teams, both in person here and by phone to those still at the staging area in Miami, of the sophisticated drug testing that would be done here.

Apparently, many who left today doubted her warning and came anyway, only to be convinced when the weightlifters were caught.

"They knew, but they were nonbelievers," Dennis said.

Roy Bergman, chief physician for the U.S. delegation, added that "our advanced medical people toured the lab and notified me, and we made the athletes fully aware. I met with the athletes and told them, but the sanctions probably reinforced their ideas."

After the announcement of the weightlifting penalties, the defecting athletes met with Dennis and told her they preferred to leave, rather than risk a positive test here that could affect their Olympic eligibility.

"If they had competed and gotten caught and banned, they would be stupid," said 400-meter runner Cliff Wiley of Baltimore. "And we don't have any stupid athletes."

He added quickly: "I'm not saying they were dirty," a word often used to describe athletes who test positively for using steroids or other illegal substances.

Bergman, cautioning against a "blanket indictment," said some of the 13 could have left for reasons other than fear of drug detection. Long jumper Randy Williams told Dennis he was going home because his wife had given birth Monday.

The others were Maryland graduate Ian Pyka and Jesse Stuart, shot putters; Paul Bishop and Greg McSeveney, discus; Dave McKenzie and John McArdle, hammer; Duncan Atwood, javelin; Mike Marlow, triple jump; Gary Bastien, decathlon; Mike Tully, pole vault; Mark Patrick, 400-meter hurdles, and Brady Crain, 4x100-meter relay.

Only McKenzie is a U.S. champion. Tully, McArdle and Bastien were third in the U.S. Championships, while the others placed from fifth to ninth at Indianapolis.

Later in the day it was announced that Chile's top cyclist, Fernando Vera, has been suspended from competition for one month for using anabolic steroids.

Vera, a medalist in the 4,000-meter pursuit at the 1979 Pan American Games, acknowledged taking anabolic steroids two months ago to help rebuild an atrophied muscle.

Use of steroids by weightlifters and weight throwers in track and field has been an international scandal for years, with athletes benefiting from the inability of testing techniques to detect them. Anabolic steroids are synthetically produced hormones that can add bulk to muscle.

The athletes' attitude has been that they need to use steroids in order to compete, because everybody else is aided by them. Dennis said that attitude has not necessarily changed.

"The athletes wanted to know yesterday, some of them, what the Olympic Committee was going to do to help them keep up with that," Dennis said. "My answer was, 'Nothing, absolutely nothing. We will help you in every way we can as athletes to excel, but in respect to banned substances we will do absolutely nothing but try to prevent it.' "

Dennis said that as chairman of the Women's Track and Field Committee of the Athletics Congress, she had led a campaign to institute drug testing at the U.S. Track and Field Championships in Indianapolis, but was outvoted by the men.

"I think the men are chicken and afraid to face reality," said Dennis, who pointed out that the U.S. would have a full women's team in action here. "The principal objection to testing was the expense, but they also said that the athletes had not had enough warning. I do not accept those reasons."

A possibly even more pressing argument was that the U.S. would have been left with a weakened team for the recent World Championships at Helsinki.

Although the situation is an embarrassment for the USOC, it affords an opportunity to end the steroid controversy, and F. Don Miller, executive director of the U.S. Olympic Committee, was quick to jump on that aspect of the situation.

"I'm being candid, when I begin to see international federations and our national governing bodies taking liberties with the testing, to sweep it under the rug, that to me is just a further hypocrisy of this entire system," Miller said.

"These things have happened and we know they've happened. Who suffers? The athletes who have been misled.

"We all know that to be eligible for the games you have to be free of chemical substances. Now that sophisticated equipment is available to do the testing properly, I think it is about time we had some integrity in this program and did it properly. It's a forewarning to everyone concerned in amateur sports.

"In my judgment, this will have a positive effect in helping to correct an insidious problem that has been going on for a long time."

In the past, U.S. doctors have been known to work with athletes, recommending a period before testing when they should stop taking steroids to avoid detection. But Bergman indicated that now there is no so-called safe period, that past use could be determined for more than a year.

"Each drug leaves footprints in the blood," Bergman said. "It is almost as positively identifying as a fingerprint. The tests can tell what drug was used, how much and when, and how the body handles it.

"It is easy for this equipment, depending on how it is set, to detect agents within a year, possibly several years."

Bergman added that the dangers of steroids far outweigh any temporary benefit they could give an athlete.

"It is not fully documented, of course, but some drugs are very dangerous and can result--and have resulted--in premature death. They can cause heart disease, tumors in the liver, impotence or sterility.

"As for performance, no one really knows what these things do. What they can do is shorten the recovery time between events, but they've never been proven to aid actual performance."

Bergman said a similar testing system had been used at the World Track and Field Championships, but that it was calibrated differently and the system here is much more sensitive. He said the same sophisticated level of testing would be used during the 1984 Olympics.