Accountant Brian Cullinan has been named by his firm PwC as the main culprit for the "series of mistakes" that led to the biggest blunder in Oscars history.
According to a statement released by the consultancy giant, Mr Cullinan and his colleague Martha Ruiz did not follow protocols "quickly enough" once "the error occurred".
On Sunday's 89th Academy Awards, La La Land was wrongly announced as best picture winner by presenters Warren Beatty and Faye Dunaway.
The error came to light while the La La Land team were still on the stage receiving the award, and amid chaotic scenes one of the La La Land producers walked up to the microphone and announced the real winner - Moonlight, directed by Barry Jenkins.
"PwC takes full responsibility for the series of mistakes and breaches of established protocols during last night's Oscars," the statement reads.
"Partner Brian Cullinan mistakenly handed the back-up envelope for Actress in a Leading Role instead of the envelope for Best Picture to presenters Warren Beatty and Faye Dunaway.
"Once the error occurred, protocols for correcting it were not followed through quickly enough by Mr Cullinan or his partner."
Mr Cullinan has been working for PricewaterhouseCoopers for 20 years, and currently serves as US board chairman and managing partner.
According to his Twitter profile, he is also in charge of "counting Oscar ballots and keeping secrets".
Before the ceremony, the accountant had posted several photos with stars on the red carpet.
PwC has been the official Academy Awards firm for more than 80 years, counting the approximately 7,000 votes by Academy members and producing the envelopes with the winners.
During the ceremony, two accountants - who are the only people to know all the winners - usually stand just off stage, monitoring the proceedings and handing out the envelopes to award announcers.
According to PwC, procedure calls for the PwC employees take the stage if an error occurs.
La La Land producer Jordan Horowitz was the one who informed the audience there had been a mistake.
"There's a mistake. Moonlight, you guys won best picture," Horowitz said.
Beatty then proceeded to explain the mix-up to the audience, saying he had been given the wrong envelope.
Shortly after the ceremony, PwC apologised to Beatty, Dunaway, Kimmel and all those involved, saying it had "failed the Academy".