Mark McGwire's confession regarding the use of steroids on Monday confirmed what had been widely speculated for years, but according to two reports, there was proof of McGwire's use of performance-enhancing substances more than 15 years ago.

Now-retired FBI agent Greg Stejskal told ESPN.com on Tuesday that Operation Equine, an investigation from 1989-93 that led to more than 70 steroid-related convictions, uncovered McGwire's use of performance-enhancing drugs in 1993 but never brought charges against the slugger because it was going after the dealers, instead of the individuals -- like Major Leaguers -- who benefited from it.

Mark McGwire

Also Tuesday, an unnamed FBI informant involved in that investigation told the New York Daily News that McGwire's use of PEDs was a lot more aggressive than he has been recounting. McGwire stated that he only used steroids in order to get healthy -- specifically the period from 1993-95, when he was limited to 178 games in three seasons. But the unnamed informant said McGwire's regimen put him closer to the "Pumping Iron" culture of the mid-1980s than the conservative HGH use and hormone therapy promoted in today's anti-aging industry, the Daily News wrote.

The Daily News reported in 2005, citing unnamed sources from the same FBI investigation, that McGwire used half a cc of testosterone cypionate every three days, which the informant described as a long-acting substance that stays in your system a long time and is "dirty" because it's easy to test positive for it. Testosterone cypionate is what individuals used to get stronger, and it was usually combined with Deca-Durabolin, the Daily News added.

In his regimen, McGwire also reportedly mixed in testosterone enanthate and the veterinary steroids Equipoise and Winstrol V -- which is known among bodybuilders to increase joint pain -- the Daily News wrote.

Dan Gwartney, a physician in Columbia, Mo., and columnist for the bodybuilding magazine Muscular Development, added that such a regimen was roughly two to four times what a patient getting testosterone-replacement therapy would get at a clinic.

"It would aid in recovery but goes above that range," Gwartney told the Daily News. "There would be performance enhancement for sports that depend on power, mass, an aggressive mindset and endurance. Whether or not that was his intent, I don't know. But that dosage would enhance performance."

Stejskal confirmed to ESPN.com that McGwire and his then-teammate Jose Canseco were among those identified in the undercover Operation Equine, but federal authorities never considered bringing charges against McGwire or other players.

"We had two sources that told us they personally had seen [McGwire] use steroids," Stejskal told ESPN.com. "Not to mention the fact that Canseco said it as well, which gives you a third source. But we knew it before Canseco said it. And we knew specifics, too. We knew this wasn't a one-time shot or an experimental thing. This guy had a regimen and stuff."