A longtime ally of Donald Trump resigned from his post leading a Department of Justice investigation into whether former law enforcement and intelligence officials conspired against the Republican president.

“Sometimes people want things done faster and that’s all you can do,” Joe diGenova told the Guardian on Thursday. He appeared to be referring to concerns among other Trump loyalists that the president’s retribution cases were not moving quickly enough.

“The bottom line is it takes a long time to make good cases, and you simply have to work at it,” he added. “All you can do is do the best you can under the circumstances.”

The departure injects instant tumult into an investigation targeting former government officials who have investigated Trump over the last decade, including in connection with Russian interference in the 2016 presidential election and his retention of classified documents at his Mar-a-Lago estate in Palm Beach, Florida.

The Florida-based investigation has proceeded in fits and starts since its inception a year ago, with its contours loosely defined, though it had shown recent signs of activity with new subpoenas demanding testimony before a federal grand jury.

While retribution by the justice department against Trump’s perceived adversaries has been one of the president’s highest priorities, diGenova’s comments hint at internal dissent and disagreement over the cases. He declined to say exactly why he resigned.

“I’m not going to make any comments about indictments or anything else,” diGenova said. “When you’ve been a prosecutor as long as I have you know how to make cases, and it takes time. And that’s just part of the process. And sometimes that works out, and sometimes it doesn’t.”

The justice department sent the Guardian a statement after this article was published: “We are grateful to Joe diGenova for his many years of service to the Department. The Attorney General thanks him for his work these past five months alongside the great team of experienced prosecutors in the Southern District of Florida, led by US Attorney Jason Reding Quiñones.”

During the first Trump administration, diGenova was a legal adviser to John Solomon, a conservative journalist who has played an official role in White House efforts to push the conspiracy allegations.

In an interview later with the New York Post, diGenova responded to criticism of his management of the investigation by saying: “If you want indictments where there’s no evidence, you have an ethical problem.”

A spokesperson for the justice department did not immediately return a message seeking comment.

The 81-year-old diGenova served as the US attorney for the District of Columbia during the Reagan administration. A close Trump ally, he was brought back to the justice department in April and given responsibility for an investigation aimed at trying to establish a conspiracy against Trump.

DiGenova had previously asked the then US attorney general, Pam Bondi, to appoint him to the job and had openly and repeatedly claimed Trump was the victim of an intelligence community conspiracy.

But five months later, no charges have been brought, and it remains unclear whether any will be, or what information has been gathered to support any basis for the idea of a plot against Trump by the government officials who investigated him.

The investigation has focused intensely on one of Trump’s chief grievances: a US intelligence community assessment that Russia had interfered on his behalf in the 2016 presidential election.

The president and some of his supporters have long decried that conclusion as a “deep state” plot to undermine the legitimacy of his win. Multiple government investigations, including by a justice department special counsel and by the department’s own inspector general, have concluded that Russia interfered in sweeping fashion to help get Trump elected.

The year-long investigation had proceeded in fits and starts well before diGenova’s appointment, which was announced soon after a career national security prosecutor who had been playing a key role in the investigation was replaced on the inquiry after raising doubts about its legal viability.

The Associated Press contributed reporting