(NBC News) - The NFL should have conducted a more thorough investigation of Ray Rice's assault of his fiancee in an Atlantic City casino elevator, the former head of the FBI said Thursday in a long-awaited report.

But the report said it found no evidence that the league saw a damning security video of the attack before it was leaked to the public in September.

Former FBI Director Robert Mueller laid out his findings in a report released online.

NFL Commissioner Roger Goodell first suspended Rice two for games, then suspended him indefinitely after the video showing Rice's cold-cocking of Janay Palmer was leaked. Goodell said he had not seen the clip before it became public. Rice, a former star running back for the Baltimore Ravens, appealed, and an arbitrator reinstated him.

Goodell appointed Mueller to investigate as the NFL was consumed by a furor over the Rice matter and other domestic-violence cases.

The report also said Mueller's investigators found no evidence that any NFL executive had received a copy of the video in April, as alleged to the Associated Press by an unnamed law enforcement official.

"We concluded there was substantial information about the incident — even without the in-elevator video—indicating the need for a more thorough investigation," Mueller's report said. "The NFL should have done more with the information it had, and should have taken additional steps to obtain all available information about the February 15 incident."

Mueller said he and his investigators reviewed "millions of documents, emails, text messages and electronic data logs."