WASHINGTON — The primary liaison between the Justice Department and the special counsel’s office is set to step down at the end of the month, after helping oversee the department’s most consequential and controversial investigation in a generation.
Edward O’Callaghan, who served as the deputy attorney general’s right-hand man, worked closely with the deputy attorney general at the time, Rod J. Rosenstein, to oversee the Russia investigation. He had primary supervisory responsibilities over the special counsel’s office, which was led by Robert S. Mueller III.
Mr. O’Callaghan, who is the top adviser to Jeffrey A. Rosen, who succeeded Mr. Rosenstein as deputy attorney general, also advised Mr. Rosenstein and Attorney General William P. Barr on how best to release the Mueller report this spring. Mr. O’Callaghan, who plans to return to private practice, played a direct role in the decision not to accuse President Trump of obstruction of justice in the report and worked directly on the process to review the report so much of it could be made public.
The Justice Department confirmed Mr. O’Callaghan’s move.
Mr. O’Callaghan “is one of the most highly regarded lawyers at the Department of Justice,” Mr. Barr said in a statement. “His dedication and tireless commitment to the work of the department is second to none.”
A veteran of the federal prosecutor’s office in Manhattan, Mr. O’Callaghan was part of a team that investigated the terrorists behind the Sept. 11, 2001, attacks.
He left the government in 2008 to become a defense lawyer and went on to work on John McCain’s presidential campaign that year. Mr. O’Callaghan was dispatched to Alaska, where he led a team that defended Sarah Palin, Mr. McCain’s running mate, from accusations that she had pushed an Alaskan official to fire a state trooper who was involved in a bitter divorce from her sister.
Mr. O’Callaghan returned to the Justice Department in 2017 as the head of the national security division, and in April 2018, he moved to the deputy attorney general’s office, which essentially runs the department’s day-to-day functions and oversees all United States attorneys’ offices. He worked behind the scenes, holding one of the department’s most powerful positions and advising the deputy attorney general on the department’s biggest investigations and most delicate policy matters.
Mr. O’Callaghan began working closely with Mr. Rosenstein nearly a year after Mr. Mueller took over the Russia inquiry into whether the Trump campaign had conspired with Russia to interfere in the 2016 election. The sensitivities around that inquiry became even more heightened as Mr. Mueller’s team began to consider whether Mr. Trump himself had tried to obstruct the investigation.
Mr. Mueller ultimately said that he had “insufficient evidence” to determine whether the Trump campaign engaged in a criminal conspiracy with the Russians, even though the campaign expected to benefit from the sabotage. The special counsel did not draw any conclusions about whether Mr. Trump obstructed justice, leaving room for Mr. Barr to clear the president of that crime.
Senior Justice Department officials had considered making Mr. O’Callaghan the United States attorney in Manhattan. But that plan petered out in recent months, in part because the current United States attorney, Geoffrey S. Berman, had no plans to leave his post, according to a person with knowledge of the matter.
This year, Mr. O’Callaghan had been in line to replace Jessie K. Liu as the United States attorney in Washington after Ms. Liu was nominated to join the Justice Department as its No. 3 official, the associate attorney general. But that plan fell through after the Senate declined to approve Ms. Liu’s nomination, according to three people who had been briefed on those negotiations.