"First, let us say that we did not do all we should have done and no amount of explanation can remedy that," Bisciotti wrote in the letter, which was posted on the team's website.
Rice and his then-fiancee, Janay Rice, were involved in a Feb. 15 altercation in an Atlantic City hotel elevator. The Ravens initially supported Rice following the incident, but terminated the running back's contract Monday after a second video from the February assault emerged.
The newly-emerged video shows Janay Palmer, now Rice's wife, being hit in the face in the elevator. Palmer then lunges at the running back before he delivers a blow that knocks her out. When the elevator doors open, Rice drags Palmer's body outside, leaving her face down on the floor, her legs still inside the elevator.
A previous video only showed the scene outside the elevator.
The letter posted on the team's website, addressed to Ravens stakeholders, begins, "You deserve an explanation."
Bisciotti wrote that following the incident, team officials spoke to Rice, Palmer, the casino management, police and prosecutors.
"In March, the prosecutor dropped the case against Janay, but elevated the charge against Ray from simple assault to aggravated assault," the letter says. "At this point, we decided to defer action until completion of the court proceedings. We stopped seeking to view or obtain a copy of the video. We halted our fact-finding. That was a mistake on our part."
But the letter indicates that the team, like the NFL, was taking its lead from how law enforcement was handling the case, and police and prosecutors had seen the full video.
"In May, the prosecutor recommended, and the judge agreed, that Ray should be accepted into a pre-trial intervention program that will eventually have the assault charge dismissed from his record, pending a year of good behavior," Bisciotti wrote in the letter.
"The police had seen video from inside the elevator," it continues. "The prosecutor and the judge, who had also seen such video, allowed Ray into the program that would eventually clear him of the assault charge."
Rice and Palmer met with NFL Commissioner Roger Goodell on June 16. Weeks later, Goodell announced that Rice would be suspended for the first two games of the season.
Seeing the video, however, "changed everything," Bisciotti said.
"It is violent and horrifying," he said. "I immediately came to the office and called a meeting with [team president] Dick Cass, [general manager] Ozzie Newsome, [coach] John Harbaugh and [senior vice president of public relations] Kevin Byrne. The meeting was relatively short. The decision to let Ray Rice go was unanimous. Seeing that video changed everything. We should have seen it earlier. We should have pursued our own investigation more vigorously. We didn't and we were wrong."
He said that Rice had "earned every benefit of the doubt" from the team because of his contributions on and off the field for six years.
"We took everything we knew and decided to support Ray Rice until we could not," he said.