Unencumbered, Mr. Burck and Mr. McGahn met the special counsel team in November for the first time and shared all that Mr. McGahn knew.
To investigators, Mr. McGahn was a fruitful witness, people familiar with the investigation said. He had been directly involved in nearly every episode they are scrutinizing to determine whether the president obstructed justice. To make an obstruction case, prosecutors who lack a piece of slam-dunk evidence generally point to a range of actions that prove that the suspect tried to interfere with the inquiry.
Mr. McGahn gave to Mr. Mueller’s investigators, the people said, a sense of the president’s mind-set in the days leading to the firing of Mr. Comey; how the White House handled the firing of the former national security adviser, Michael T. Flynn; and how Mr. Trump repeatedly berated Mr. Sessions, tried to get him to assert control over the investigation and threatened to fire him.
Despite the Trump lawyers’ insistence that cooperation would help end the inquiry, the investigation only intensified as last year came to a close. Mr. Mueller had charged Mr. Trump’s former campaign chairman and his deputy and won guilty pleas and cooperation agreements from his first national security adviser and a campaign adviser.
Mr. Dowd said that cooperation was the right approach but that Mr. Mueller had “snookered” Mr. Trump’s legal team. The White House has handed over more than one million documents and allowed more than two dozen administration officials to meet with Mr. Mueller in the belief that he would be forced to conclude there was no obstruction case.
“It was an extraordinary cooperation — more cooperation than in any major case — no president has ever been more cooperative than this,” Mr. Dowd said, adding that Mr. Mueller knew as far back as October, when he received many White House documents, that the president did not break the law.
As the months passed on, it became apparent that Mr. McGahn and Mr. Burck had overestimated the amount of thought that they believed the president put into his legal strategy. Rather than placing the blame on Mr. McGahn for possible acts of obstruction, Mr. Trump has yet to even meet with the special counsel, his lawyers resisting an invitation for an interview. Mr. McGahn is still the White House counsel, shepherding the president’s second Supreme Court nominee, Brett M. Kavanaugh, through the confirmation process.
Mr. Mueller, armed with Mr. McGahn’s account, is still trying to interview witnesses close to the president. But the White House has a new lawyer for the investigation, Emmet T. Flood, who has strong views on privilege issues. When the special counsel asked to interview Mr. Kelly, Mr. Flood contested the request, rather than fully cooperate.