The 48-year-old left-hander, six times an Open runner-up in the only major he has never won, sent a text message blaming his anger and frustration on a difficult day for his major rules violation.
"I know this should've come sooner, but it's taken me a few days to calm down," Mickelson wrote to the website.
"My anger and frustration got the best of me last weekend. I'm embarrassed and disappointed by my actions. It was clearly not my finest moment and I'm sorry."
He was given a two-stroke penalty for the violation but many suggested he should have been disqualified for the move.
The report said Mickelson's wife, Amy Mickelson, said her husband had offered to disqualify himself from the event to USGA president Mike Davis.
Mickelson eventually shared 48th place at 16 over par for the tournament.
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