Sports Illustrated reports it was David Eger, a rules expert who has worked for the USGA and the PGA Tour.
Eger says he was watching the Masters when he replayed the 15th hole to see how Woods made bogey. Woods hit the pin with his third shot, which bounced back into the water. He dropped from the same area, hit to 4 feet and made bogey.
"I could see there was a divot -not a divot, a divot hole - when he played the shot the second time that was not there the first time," Eger told the magazine. "I played it again and again. I could see that the fairway was spotless the first time he played the shot and there was that divot hole, maybe 3 or 4 feet in front of where he played after the drop."
Eger, who now plays on the Champions Tour, said he called Mickey Bradley, one of the PGA Tour rules officials who was working at Augusta National.
Bradley was no longer at the course, but he called Fred Ridley, chairman of the Masters competition committee. Bradley also said he sent a copy of Eger's text to Ridley and to Mark Russell, the tour's vice president of competition who serves on the Masters committee.
Woods eventually was given a two-shot penalty for the illegal drop, but he was not disqualified for signing an incorrect scorecard.
Ridley said at the Masters he reviewed the video and didn't see a violation, so he chose not to bring it up to Woods before he signed his card after Friday's round.
Woods later said he purposely dropped it a few yards back to avoid hitting the pin, and more questions were brought to Ridley's attention later that night. Ridley met with Woods before the third round and assessed the penalty.
The magazine said Ridley responded by text message to Bradley that Woods' drop was closer than Eger's estimate of 3 to 4 feet and to look at it closer would be "splitting hairs." That's why he chose not to bring it up with Woods before he signed his card.
Ridley cited Rule 33-7 to explain why Woods was not disqualified. The rule says disqualification in exceptional cases can be waived by the committee's discretion. In this case, Ridley said by not talking to Woods about the violation before he signed his card, that was ample reason not to disqualify him.
Also on Tuesday, the USGA and Royal & Ancient released a lengthy review in which they concluded the Masters "reasonably exercised its discretion" under Rule 33-7 to waive disqualification.
The USGA and R&A said Rule 33-7 that allows a committee to waive disqualifications has been around for 60 years. They said they would review the "exceptional situation" involving Woods at the Masters, assess potential implications for other situations and decide whether the rules need to be adjusted.
Eger was director of tournament administration at the PGA Tour for 10 years until joining the USGA as senior director of rules and competition from 1992 to 1995. He returned to the PGA Tour as vice president of competition until 1996.
As a player, Eger won the 1988 US Mid-Amateur and the North and South Amateur in 1991. He also played on three Walker Cup teams, reached the semi-finals of the USAmateur twice and has four victories on the Champions Tour.