Arkansas athletic director Jeff Long made the announcement at a 10 p.m. press conference after a state police report showed that Razorback athletic department employee Jessica Dorrell was also on his motorcycle.
Petrino called Long at 3:12 p.m. on Thursday to inform him that the coach had not told the truth about the wreck.
He also told Long that he had a "previous" inappropriate relationship with the 25-year old Dorrell, a former Lady Razorback volleyball player that was recently hired as the Student-Athlete Development Coordinator.
"At that time Coach Petrino shared with me some information that he had not previously shared," Long said. "That information was that there was another passenger on the motorcycle and that passenger was an employee of ours in the athletic department at the University of Arkansas."
Long's decision came after meeting with Petrino at around 7:15 p.m. on Thursday night and after a brief conversation with Dorrell, who he said told him she was not injured.
Petrino is married while Dorrell is engaged to Josh Morgan, the director of swimming and diving operations at Arkansas.
"After our meeting I asked him to remain in the facility while I considered the information that I had," Long said. "At about 8:30 I informed Coach that given this information and the freshness of it and the short period of time I have had the information that I was going to put him on administrative leave while I take the time necessary to review this personal matter."
The athletic director did not rule out further discipline and also did not say that Petrino's job was definitely safe.
"I'm at the beginning of the review," Long said. "I don't know what I'm going to find."
Long announced that assistant head coach Taver Johnson would take over as the acting head coach while he conducts the review.
While Petrino was not forthcoming with his bosses about the accident, he was with the state police on Tuesday according to police spokesman Bill Sadler.
"Coach Petrino was as cooperative as anybody that we could ever hope to encounter following the traffic crash," Sadler told the Associated Press. "He was very forthcoming with information when approached by the trooper who made the initial contact Tuesday afternoon."
Petrino suffered four broken ribs and damage to his C2 veterbrae and wore a neck brace to his press conference on Tuesday.
A press release earlier in the week from the University of Arkansas stated that there was no one else involved in the wreck.
Petrino was not at the press conference, but did release a statement before the press conference started.
"The state police report today provides an accurate description of my accident, which includes details that had not publicly come to light prior to the report being issued," Petrino said. "I regret that I have not publicly acknowledged a passenger on the vehicle. ??
"I have been in constant pain, medicated and the circumstances involving the wreck have come out in bits and pieces," Petrino added. "That said I certainly had a concern about Jessica Dorrell's name being revealed. In my press conference, I referred to her simply as ‘a lady'. My concern was to protect my family and a previous inappropriate relationship from becoming public.
"In hindsight, I showed a serious mistake in judgment when I chose not to be more specific about those details," Petrino continued. "Today, I've acknowledged this previous inappropriate relationship with my family and those within the athletic department administration.
"I apologize to my wife, Becky, and our four children, Chancellor (David) Gearhart, Jeff Long, the Board of Trustees, University administration, my coaching staff, student-athletes and the entire state of Arkansas.
"I have been humbled by the outpouring of concern and get-well wishes. I apologize to the Razorback Nation for the attention my actions have brought to the University of Arkansas and our program.
"I will fully cooperate with the University throughout this process and my hope is to repair my relationships with my family, my Athletic Director, the Razorback Nation and remain the head coach of the Razorbacks."