Attorney General William Barr said he "couldn't say" when special counsel Robert Mueller knew there was no collusion between the Trump campaign and Russia.

He was asked about it during a Fox News interview that aired Friday.

"Can you say when Bob Mueller knew there was no collusion with Russia?" asked Fox News anchor Bill Hemmer.

"No, I couldn't say that," Barr replied.

"No date given?" Hemmer asked.

"No, I couldn't say when he knew," Barr said.

Mueller was appointed to lead the investigation into Russian interference in the 2016 election in May 2017, eight days after President Trump fired former FBI Director James Comey.

The Justice Department released Mueller's 448-page report with redactions last month which noted the term "collusion" has no legal application. Instead, Mueller's team focused on analyzing the prospect of criminal conspiracy.

Mueller determined that Russians meddled in the 2016 election and had contact with individuals affiliated with the Trump campaign. He did not however charge any Trump campaign official or any other U.S. person with participating in conspiracy.

"That is because the investigation did not identify evidence that any U.S. person who coordinated or communicated with the IRA knew that he or she was speaking with Russian nationals engaged in the criminal conspiracy," Mueller wrote, referring to the Internet Research Agency — a company based in St. Petersburg that was accused by the U.S. government of running an online troll farm to disrupt the U.S. election.