PwC's statement added: "For the past 83 years, the Academy has entrusted PwC with the integrity of the awards process during the ceremony, and last night we failed the Academy."

The Academy also broke its silence on Monday evening.

"We salute the tremendous grace they displayed under the circumstances," the statement said. "To all involved --  including our presenters Warren Beatty and Faye Dunaway, the filmmakers, and our fans watching worldwide  -- we apologize."

The Academy added: "We have spent last night and today investigating the circumstances, and will determine what actions are appropriate going forward. We are unwaveringly committed to upholding the integrity of the Oscars and the Academy of Motion Picture Arts and Sciences."

The statements come after revelations that Cullinan was tweeting shortly before the mistake was made.

At 9:05 p.m. on Sunday, Cullinan sent a now-deleted tweet from his personal account that showed fresh-off-the-stage winner Emma Stone smiling with her new Oscar in hand.

"Best Actress Emma Stone backstage! #PWC," Cullinan wrote.

The tweet is viewable in a cached page on Google.

The social media post came just minutes before Dunaway and Beatty were inadvertently given the envelope containing the already announced best actress results.

Cullinan has not responded to CNN's request for comment as of Monday evening.

PwC quickly apologized on Sunday night and said they were "investigating how this could have happened."

In the history of the Oscars, only one similar event has taken place.

Back in 1964, Sammy Davis Jr. was handed the wrong envelope when presenting the best musical score (adaptation or treatment) category and accidentally announced the winner for original musical score.

He corrected himself on stage after he was told he announced a winner who was not nominated in the category he was supposed to be presenting.

PwC has several measures in place meant to prevent such occurrences.

It's worth noting, however, that this year's envelope design was changed from gold paper with white labels to red paper with gold lettering -- considerably harder to read in dim backstage lighting.

To the cameras capturing the action on stage, the misstep was clear. Photos from Sunday, examined in detail in the hours after the show, confirmed that Dunaway and Beatty had the wrong envelope on stage.

As an additional safety measure, Ruiz and Cullinan memorize the names of the winners. This is so they can be quick to respond in case the wrong name is read or share the winner with presenters in case something happens to the envelopes.

Three "La La Land" producers spoke on stage before the error was announced to the Oscars audience.

"Moonlight" ended up winning three Oscars on Sunday night. "La La Land" earned five.