The American anti-doping authority said in its report the highest-profile track and field coach in the world had attempted to use prohibited methods on multiple athletes.
With Nike-backed athletics coach Alberto Salazar slapped with a four-year ban for doping, the US sportswear giant risks being caught up in the scandal -- its CEO is even quoted in the suspension ruling.
The American anti-doping authority USADA -- citing experiments with testosterone, fat-burning amino acid injections, and falsified medical documents -- said in its report the highest-profile track and field coach in the world had attempted to use prohibited methods on multiple athletes.
Mentioned in the document -- prepared by an independent panel for USADA -- was Mark Parker, the CEO of Nike, which has sponsored Salazar for decades.
Parker was copied on several emails about research done by Salazar and the Nike Oregon Project (NOP), a group created in 2001 by the three-time New York Marathon winner.
The alleged program was for athletes competing at 5,000 and 10,000 meters but not for sprint races, according to the agency.
In a 2011 email to Parker, Salazar explains he had given one of the NOP coaches a test injection of a liter of an amino acid and dextrose (glucose) mixture -- a dose clearly above what would be allowed under World Anti-Doping Agency (WADA) regulations.
In another email to Parker two years later, Jeffrey Brown, a doctor who worked with the NOP, described experiments with testosterone gel.
Parker responded to Brown, "It will be interesting to determine the minimal amount of topical male hormone required to create a positive test."
In an email obtained by AFP after it was sent to employees on Tuesday, Parker wrote "I had no reason to believe the test was outside any rules," as the experiment was carried out with a medical doctor.
These tests were done in response to Salazar's concern that an athlete might be sabotaged by someone secretly contaminating them with the gel, Parker said.
"As a runner, I was appalled and shocked that this could be the case," the CEO said.
"Nike did not participate in any effort to systematically dope any runners ever; the very idea makes me sick," Parker wrote.
USADA Chief Executive Travis Tygart was scathing, telling German broadcaster ZDF that Salazar had used athletes as "guinea pigs".
He added, "I hope Nike sees this as a wake-up call."
Salazar's best-known athlete is Britain's Mo Farah, who won four gold medals at the 2012 and 2016 Olympics at 5,000m and 10,000m.
Farah, who has never tested positive, said in response to Salazar's suspension he left the NOP in 2017 and that he has "no tolerance for anyone who breaks the rules or crosses the line".
Swooshes on timebombs
Nike stock was down 1.33 percent at $91.05 midway through trading Wednesday, having hit its highest price ever on Monday. The Dow overall was down 1.85 percent.
The brand has posted glowing results for several quarters, bolstered by its reorientation towards online sales and its performance in China.
Could the Salazar slam the brakes on its upward movement?
"Nike's history is full of supporting dopers, of doped federations," former runner Lauren Fleshman, a Nike-sponsored athlete for nine years until 2012, tweeted on Tuesday.
"They look the other way even when it's clear to everyone else that something is rotten," she said.
Nike continued to officially support cyclist Lance Armstrong in 2012 immediately after the publication of USADA's report showing damning evidence of doping against him and his team -- before dropping him a few days later.
In June 2016, Nike maintained its contract with tennis player Maria Sharapova, who was suspended for two years for doping.
The clothing brand also stood by basketball player Kobe Bryant, accused of rape in 2003, and golfer Tiger Woods, during an adultery scandal in 2009.
And it continued its association with US sprinter Justin Gatlin, who served a ban for doping before returning to win world titles.
In September 2018, Nike made waves when it released an advertising campaign featuring US football player and activist Colin Kaepernick, criticized for kneeling during the US national anthem at games in protest at racism.
"Nike is having a public reckoning right now," tweeted Fleshman, who competed at 5,000m in three world championships.
She also recalled that Nike was revealed to have been penalising pregnant athletes, a policy it revised in May after coming under pressure from track veteran and new mother Allyson Felix.
And Fleshman criticized their ads, including one featuring Serena Williams and other female athletes in light of the recent scandal.
"If you make ads about moms kicking ass but you suspend pregnant women without pay while preventing them from making money elsewhere - If you make ads about the purity of sport while funding the underbelly that erodes it - that's a problem."