The Nevada commission collected hair samples from Alvarez on March 29 and they were tested at the Sports Medicine Research & Testing Laboratory, a WADA-accredited lab in Salt Lake City, Utah, Bob Bennett, the commission executive director, told ESPN.

The results document, which was provided to ESPN, was returned on April 20 with a single note in the section reserved for analysis details: "Hair sample tested for clenbuterol. Clenbuterol was not detected."

The negative hair follicle test is notable because while clenbuterol is detectable in urine for less than a week after it is ingested, the drug can be detected in hair for several months.

That would appear to lend credibility to Alvarez's assertion that he was positive for trace amounts of clenbuterol in random urine tests conducted by the Voluntary Anti-Doping Association in his hometown of Guadalajara, Mexico, on Feb. 17 and Feb. 20 because he consumed a small amount that was present in the meat he ate as opposed to taking larger doses of the substance as part of a doping program that would be more likely to remain in his hair follicles for months after he had stopped taking it.

According to Dopeology.org, which extensively covers doping in professional cycling, "Clenbuterol, like many other substances, accumulates in very small amounts in hair follicles. If an athlete has taken the substance over a period of time in the past, his/her hair test might return a positive for a period of six months or longer. Conversely, if the clenbuterol resulted from a single incident, in which the subject had eaten meat for example, a hair test would almost certainly be negative."

"From the beginning, Canelo has insisted that he accidentally ingested clenbuterol from eating tainted meat," Eric Gomez, president of Golden Boy Promotions, Alvarez's promoter, told ESPN. "The fact this NSAC-required hair follicle test came back entirely negative for any traces of clenbuterol should lay to rest any suspicion that he was intentionally taking a banned substance. Canelo is looking forward to getting back into training and returning to the ring in September. He thanks all of his fans for sticking by him and believes this test proves once and for all that he is a clean fighter."

Contaminated meat in Mexico is a long-standing issue for athletes in Mexico, where farmers often include the substance in cattle feed because it helps reduce fat and increase lean muscle mass.

"They found traces of clenbuterol in his system and if he was using it (to dope) there would probably still be enough in his system that it would show up in his hair," Golden Boy Promotions CEO Oscar De La Hoya told ESPN. "We did the hair test to prove this was accidental. Canelo is telling the truth about the meat contamination and he went far and beyond what he had to do by agreeing to have this hair test done so he can keep on proving that he is an innocent fighter.

"Meat contamination is a problem in Mexico. It happened with the national soccer team and it has happened with several athletes in Mexico. I believe what Canelo wants to do now is keep proving until his career is over, 10 or 15 years from now, that he is a clean fighter and an innocent fighter and that he did not do this purposely whatsoever."

Alvarez (49-1-2, 34 KOs), 27, was scheduled to face Golovkin in a rematch of their controversial September draw on May 5 in a major HBO PPV fight at the T-Mobile Arena in Las Vegas. However, because of the two failed drug tests, and facing a suspension from the commission, Alvarez withdrew from the bout on April 3.

Nevada commission rules call for a suspension when a banned substance is found in an athlete's system, regardless of how it got there, so Alvarez was suspended for six months, per commission guidelines, in a 5-0 vote of the panel at a hearing on April 18.

"We initiated the test to be as comprehensive as possible but knowing it's very difficult, according to the experts," Bennett said. "It's a difficult process to be able to confirm whether there is clenbuterol in the hair follicles, but I talked to (SMRTL president and laboratory director) Dr. (Michael) Eichner and he said they'd run the test even though it is even more difficult to run on light-colored hair. But we sent them the samples and the tests came back negative."

Bennett said it was the first time the commission had ever asked for a hair follicle test on a combatant, but the commission felt it was important to get as much information as possible about Alvarez's case given the magnitude of the rematch with Golovkin. In the days following Alvarez's positive tests he was tested a few more times and each came back negative.

However, since the fight was canceled, Alvarez has been criticized for not continuing with testing even though he could have VADA continue a testing program without a fight scheduled. According to Golden Boy, he will soon begin a random testing program. Alvarez's suspension will be lifted on Aug. 17, six months from the day of the first positive test, and De La Hoya said the former junior middleweight and middleweight world titleholder will return to the ring on Sept. 15, on the weekend of Mexican Independence Day.

"He will fight in September and he will continue to test and continue to abide by all the rules and regulations," De La Hoya said.

"Canelo is ready and willing and able to do whatever the commission asks. We're waiting to hear back from VADA on when he can start testing again. Canelo is willing to pay for the program. He is willing to do whatever it takes to clear his name, including take a lie detector test. We're going to do one. He wants to prove he is a clean fighter."

Golovkin (37-0-1, 33 KOs) is scheduled to face late substitute opponent Vanes Martirosyan (36-3-1, 21 KOs) on May 5 (HBO, 11 p.m. ET/PT) at the StubHub Center in Carson, California. If he wins, the rematch with Alvarez could be Sept. 15, but De La Hoya would not commit to the fight.

"We first have to see what happens with Golovkin on May 5 and then we'll take it from there, but Canelo will definitely fight in September," De La Hoya said. "Golovkin is the priority, but we don't know what will happen on May 5. But on our side, Canelo will fight in September."