Ukwachu on Thursday was found guilty of sexual assault of a then-Baylor soccer player in 2013.
Briles said Friday that he spoke with Petersen personally in 2013 and was not informed of any prior violent incidents prior to accepting Ukwuachu's transfer to Baylor.
"No mention of anything beyond Sam being depressed and needing to come home," Briles said. "So that was our information. And that's what you go by."
Petersen, who is now the coach at Washington, issued a statement later Friday that said that just wasn't the case.
"After Sam Ukwuachu was dismissed from the Boise State football program and expressed an interest in transferring to Baylor, I initiated a call with coach Art Briles," Petersen's statement said. "In that conversation, I thoroughly apprised Coach Briles of the circumstances surrounding Sam's disciplinary record and dismissal."
The woman, who has since transferred, testified that Ukwuachu assaulted her at his apartment and that she yelled "no" and screamed during the assault, according to reports. She also testified that Ukwuachu told her, "This is not rape," and asked her whether she was going to call the police.
Briles called Ukwuachu's conviction "unfortunate for everybody concerned," and he reiterated that the defensive end had been taken off the roster before he ever played or practiced with the team.
"Our timeline was followed by what the standards were here," Briles said. "When the incident happened, he's off the roster. Never played a down for us. So it's a very unfortunate situation for all concerned. That's all I've got to say about it."
Ukwuachu is seeking probation and faces up to 20 years in prison. His sentencing process continued Friday morning at the 54th State District Court in Waco.
During the trial, a former girlfriend at Boise State testified that Ukwuachu punched her in the head several times, choked her, physically restrained her from leaving, and had a reputation for having a violent temper.
Ukwuachu, who was dismissed from Boise State in May 2013 for an unspecified violation of team rules, denied those allegations. He was ineligible to play for Baylor in the fall of 2013 due to NCAA transfer rules and suspended for the 2014 season.
When asked specifically if Boise State had informed Baylor of Ukwuachu's disciplinary record, Briles was emphatic in his denial.
"No. No. That's not true," Briles said. "Lord, no. No, there's no truth. Find out who informed us and talk to them, please."
Baylor investigated the sexual assault but determined there was not enough evidence to proceed and was going to allow Ukwuachu to play before the district attorney indicted him, according to the Waco Tribune. The school also was prepared to let Ukwuachu return to the team if he had been found not guilty, his attorney told the Tribune before the trial.
Briles, who spoke with reporters for four minutes before practice Friday morning, said he had no involvement in Baylor's investigation into the matter.
Boise State's decision to deny a waiver that would've allowed Ukwuachu to play in 2013 was not a red flag, Briles said, because schools rarely grant permission for immediate eligibility during typical transfer processes.
Briles defended his history of giving transfer players a second chance at Baylor and acknowledged players with lesser offenses at other schools have succeeded in his program.
"We're always going to make sure that a guy is worthy of that opportunity, and always have," Briles said.
But the head coach said he would not bring a player into his program knowing that player had a prior conviction or record of domestic violence.
"Nobody is going to do that -- bring somebody in with a prior conviction or really an allegation," Briles said. "Like I said, we made our decision on the knowledge we had two years ago."
Questions arise in Baylor player convicted of sexual assault
Joe Schad discusses the news that Baylor DE Sam Ukwuachu was found guilty of sexually assaulting a then-Baylor soccer player in 2013 and how the school handled his transfer from Boise State, where he was dismissed in May 2013 for violation of team rules.
Baylor's Ukwuachu convicted of sexual assault
Michelle Beadle and Ben Lyons react to the news that Baylor DE Sam Ukwuachu was found guilty of sexually assaulting a then-Baylor soccer player in 2013.