Justice department officials were told William Barr had been invited to meet them last summer on the same day he submitted an “unsolicited” memo that heavily criticized special counsel Robert Mueller’s investigation into obstruction of justice by Donald Trump.

Barr, who was a private attorney at the time, met the officials for lunch three weeks later and was then nominated to serve as Trump’s attorney general about six months later.

The revelation about the meeting, which was arranged by Steve Engel, the head of the Office of Legal Counsel at the Department of Justice, and which has not previously been publicly disclosed, raises new questions about whether the White House’s decision to hire Barr was influenced by private discussions he had about his legal views on Mueller’s investigation.

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Barr announced last month he had concluded there was “not sufficient” evidence found in the special counsel’s investigation to establish that Trump had committed obstruction of justice. He made the decision, he wrote in a letter to Congress, after consulting with Engel – who is a legal adviser to the White House – and Rod Rosenstein, the deputy attorney general.

Barr’s decision is controversial because, months before he was hired, he sent a 20-page memo to two top officials at the DoJ – Engel and Rosenstein – that called Mueller’s obstruction theory “fatally misconceived” and “legally insupportable”. Barr, who served as attorney general under George HW Bush, said he had written the memo in order to “make sure that all of the lawyers involved carefully considered the potential implications of the theory” that Mueller appeared to be pursuing.

Unnamed officials told the Wall Street Journal, which first reported the memo’s existence, that the memo played no role in the decision to hire Barr.

In written answers to questions posed by senators as part of his confirmation hearing, Barr said he had provided copies of his memo to Rosenstein and Engel on 8 June 2018. He said he had discussed his legal opinions with Rosenstein at lunch in early 2018 and then later, on a separate occasion, he briefly discussed his views with Engel. He then said in written answers that after writing the memo: “There was no follow-up from any of these Department officials”.

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But a person with knowledge of the matter said that Engel told justice department staff on June 8 last year that an invitation had been extended to Barr – the day the memo arrived at the justice department – for a “brown bag” lunch, in which he was invited to speak to justice department staff.

The lunch then occurred on 27 June.

A spokeswoman for the DoJ confirmed that the lunch occurred. “The timing was coincidental and the memo was not discussed,” the spokeswoman said.

“OLC regularly brings back the former heads of OLC (as do other divisions) to eat with the new team and share experiences from their time at OLC,” she added.

After publication, the DoJ said the invitation was sent to Barr on 1 June and that Barr had replied and accepted on 2 June.

The Barr luncheon was, however, not an entirely routine affair. While brown bag lunches had been a tradition at DoJ in the past, a person with knowledge of the matter said the Barr lunch was meant to kickstart the tradition again, after two years in which no such lunches had occurred.

The DoJ spokeswoman initially disputed that account and promised to give the Guardian a list of names and dates of other former officials and notable individuals who had attended such “brown bag” luncheons before Barr. But the spokeswoman then did not provide any further information.

“This revelation adds yet another data point that suggests Barr’s outlandish memo signaled he would protect Mr Trump even on highly dubious or erroneous legal grounds, and that he was swept into the administration on that basis,” said Ryan Goodman, a law professor at NYU and former special counsel at the Department of Defense.

Facebook Twitter Pinterest The deputy attorney general, Rod Rosenstein, right, has said that Barr’s memo had ‘no impact’ on the special counsel’s investigation. Photograph: Yuri Gripas/Reuters

While Mueller’s investigation was believed to be focused on Trump’s decision to fire James Comey, who was the head of the FBI and was conducting an investigation into a possible conspiracy between Russia and the Trump campaign, Barr argued in his memo that a true act of obstruction had to involve explicit destruction of evidence. Such acts might include witness tampering or other deliberate acts to impair the integrity of evidence.

The DoJ has played down the significance of Barr’s June memo. Rosenstein told the WSJ that Barr had not sought or received any non-public information regarding the ongoing investigation, and that Barr’s memo had “no impact” on the investigation.

A person with knowledge of the matter said that any discussions between Barr and Engel would be relevant because it would have informed the DoJ, and possibly the White House, of Barr’s views on executive power.

Engel has kept a relatively low profile in his role at the Office of Legal Counsel. He served on the Trump campaign transition team and is known to be a close friend of the supreme court justice Brett Kavanaugh.

Engel does not serve as a personal attorney to Trump. In his capacity at the OLC he offers advice to the White House on the scope of their powers, sometimes offering legal justification for certain White House actions.

Trump recently told Senate Republicans that he had great affection for Barr.

“I love the AG. He works fast. I love this guy. You told me I would,” he told the group, according to a report in the New York Times.

• Editor’s note: this story has been updated to clarify that justice department staff were told of Barr’s invitation to meet them on 8 June. Post-publication the justice department told the Guardian that an invitation to Barr had been extended on 1 June.