The Olympic champion was arrested for the second time on DUI charges early Tuesday in his hometown of Baltimore, another embarrassment for a swimmer who came out of retirement this year with his sights set on competing at the Rio Games.
Phelps issued an apology that sounded very familiar to the ones he made after a drunken-driving arrest a decade ago, as well as when a British tabloid published a photograph in 2009 that showed him using a marijuana pipe.
"I understand the severity of my actions and take full responsibility," Phelps said in a statement. "I know these words may not mean much right now but I am deeply sorry to everyone I have let down."
Maryland Transportation Authority police charged the 18-time gold medallist after officers said he was caught speeding and failed field sobriety tests.
The arrest came about a month after the 29-year-old Phelps won three golds and two silvers at the Pan Pacific Championships in Australia, setting himself up to compete at next summer's world championships and at Rio in 2016.
It's too early to say if Phelps might face sanctions from USA Swimming, which took no action after his 2004 arrest but suspended him from competition for three months over the pot picture.
"The news regarding Michael Phelps and his actions are disappointing and unquestionably serious," the national governing body said in a statement. "We expect our athletes to conduct themselves responsibly in and out of the pool."
The U.S. Olympic Committee had a similar reaction. CEO Scott Blackmun said the organization was "surprised" by Phelps' arrest and "disappointed on a number of fronts."
Phelps was charged with driving under the influence, excessive speed and crossing double lane lines in the Fort McHenry Tunnel on Interstate 95 in Baltimore, according to the Maryland Transportation Authority.
If convicted on the DUI charge, he would face a maximum penalty of a year in jail, a $1,000 fine and the loss of his driver's license for six months. Under Maryland law, the latest case is not considered a second offence because his first DUI conviction occurred more than five years ago.
Phelps could face the wrath of his sponsors, though there was no immediate word of any company planning to drop him.
"It's too early to tell," said Don Rockwell, the CEO of Phelps' new swimsuit sponsor, California-based Aqua Sphere. "For the most part, we're supportive. We just need to wait and see what happens. This is not a deal-breaker for us, unless we find out something else that happened."
Phelps also has deals with Subway, Under Armour, Omega and Master Spas.
In early August, Phelps announced he was ending his long relationship with Speedo to sign the deal with Aqua Sphere. Just last week, according to Rockwell, company officials were in Baltimore working with Phelps on the sizing of his new suit, which he can begin wearing at meets starting Jan. 1.
First, he must deal with a more serious issue.
A Maryland Transportation Authority police officer was using radar about 1:40 a.m. when Phelps' white 2014 Land Rover came through the tunnel at 84 mph in a 45-mph zone, the agency said in a statement.
The officer stopped Phelps just beyond the tunnel's toll plaza.
"Mr. Phelps was identified as the driver by his driver's license and appeared to be under the influence," the statement said. "He was unable to perform satisfactorily a series of standard field sobriety tests."
Kelly Melhem, a spokesperson for the transportation agency, said department vehicles are equipped with in-car video recording devices. Officials were trying to determine if there was footage of Phelps' arrest, which could be used as evidence if the case goes to trial.
The statement said Phelps was co-operative during his arrest. He was taken to an authority station and later released.
After the London Olympics two years ago, Phelps followed through on his long-stated plan to retire, having won twice as many golds as anyone else and 22 medals overall.
Phelps returned to competition in April and set his sights on competing at the Rio Games, which would be his fifth Olympics.
His first DUI arrest came in 2004 on the Eastern Shore of Maryland. Phelps received 18 months' probation, a $250 fine and was required to deliver a presentation on alcohol awareness to students at three high schools.
"I recognize the seriousness of this mistake," he said at the time. "I've learned from this mistake and will continue learning from this mistake for the rest of my life."
Phelps wasn't charged after the picture came to light of him using a marijuana pipe at a party in South Carolina. He did accept the suspension from USA Swimming for what he called "bad judgment" and "a mistake I won't make again."
One of his major sponsors, Kellogg Co., dropped him almost immediately. Subway stayed with him and remains one of his biggest backers.
Phelps' comeback is going well.
"We accomplished everything we wanted to," he said after the Pan Pacs. "We were able to find out some of the things I need to improve on over the next year, and things I want to improve on."
His biggest issue at the moment is outside the pool.
Associated Press Writers Juliet Linderman and Amanda Kell in Baltimore contributed to this report.
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