Brooks Koepka believes Patrick Reed was fully aware of his actions when he flattened out a bunker to improve his lie during the Hero World Challenge in December, saying he was "building sand castles".
Reed was penalised two strokes for his actions on the 11th hole at the Albany Golf Club in The Bahamas in December, as video footage showed him twice hacking away sand in a bunker during practice swings.
Golf's rules state that players cannot improve their ability to play a shot by "removing or pressing down sand or loose soil".
The American made a bogey but was subsequently handed a two-stroke penalty upon the completion of his round.
After accepting the penalty, Reed defended his actions, insisting he had not intended to move the sand.
However, world number one Koepka is convinced Reed knew what he was doing.
"Yeah. I don't know what he was doing, building sand castles in the sand, but you know where your club is," Koepka said in an interview on SiriusXM PGA Tour Radio when asked if Reed had cheated.
"I took three months off and I can promise you I know if I touch sand. If you look at the video, obviously he grazes the sand twice and then he still chops down on it."
"It goes on a little bit more than people think," Koepka added of players improving lies.
"I haven't opened my mouth. But now if I saw it, just because of where I'm at in the game, the stature that I have, I would definitely say something."