Federal prosecutors are struggling to put together a criminal case against John O. Brennan, the former C.I.A. director, over his agency’s response to Russian election interference in 2016, according to senior administration officials.
The development is likely to anger President Trump and his Republican allies, who have long had Mr. Brennan, a persistent critic, in his cross hairs. Senior Trump administration intelligence officials have also harshly denounced Mr. Brennan’s response to the investigation. In July, John Ratcliffe, the current C.I.A. director, issued a criminal referral of Mr. Brennan, accusing him of lying to Congress.
That move reflected a broader campaign by Mr. Trump to pursue his perceived political enemies, as he has openly pressured the Justice Department to bring cases against them. But in recent weeks, federal prosecutors in Maryland and Virginia have failed to do so, prompting the ouster of at least one U.S. attorney, in the Eastern District of Virginia.
The stalled effort against Mr. Brennan, in the Eastern District of Pennsylvania, would be the most recent setback for such prosecutions.
Senior administration officials said on Wednesday that prosecutors believed they were making progress in the investigation of Mr. Brennan. But the decision last month by Tulsi Gabbard, the director of national intelligence, to revoke the security clearances of current and former national security officials has deeply hampered the inquiry.
The officials said a number of the people stripped of their clearances were likely to be interviewed by Justice Department prosecutors over their involvement on the intelligence community assessment of Russian influence. At least three officials, all of whom worked at least indirectly on the assessment, lost their jobs after their clearances were revoked.
Those firings took many senior intelligence officials by surprise, and were done over the objection of at least one agency head.
But other officials denied that. The Office of the Director of National Intelligence has provided huge amounts of material to prosecutors, they asserted, and the most important potential witnesses were not among those who had their security clearances revoked. These officials added that prosecutors, struggling to make a case, were looking for a scapegoat.
A spokesman for the Justice Department said it was committed to working closely with Ms. Gabbard’s agency to end what the administration calls the “weaponization” of intelligence and law enforcement by the previous administration.
Axios earlier reported that the revocations had hobbled the Justice Department inquiry.
Ms. Gabbard said in August that the revocations, which included a number of people who worked on the intelligence assessment, were in response to Mr. Trump’s orders, part of a wider purge of national security officials.
A month earlier, Mr. Ratcliffe, a sharp critic of Mr. Brennan, released a tradecraft review of that assessment before making a criminal referral to the F.B.I. Officials said that the review suggested that Mr. Brennan may have misled Congress about the role that a discredited dossier assembled by a former British spy had played in the assessment.
The revocations complicated the investigation of Mr. Brennan, the senior administration officials said, because it swept up many people involved in the assessment, including those with concerns about how it was handled. Some administration officials said they believed that the officials whose clearances were revoked would be unlikely to cooperate with prosecutors. Others said they believed their credibility with juries might be impaired.
But the revocations did not include the C.I.A. officers who worked most closely with Mr. Brennan and wrote the bulk of the classified report that was issued in December 2016, an unclassified version of which was released in January 2017, two weeks before Mr. Trump took office for his first term.
Mr. Brennan has repeatedly denied any wrongdoing, and his public comments on the dossier and the intelligence assessment have been consistent. His actions were examined by multiple congressional committees and a special counsel appointed during the first Trump administration. Legal experts have raised questions about whether any prosecution of Mr. Brennan would be possible.
Current and former intelligence officials involved in the assessment are skeptical that Mr. Brennan violated any law, but some believe he did mishandle parts of the assessment. In hindsight, some have said, the dossier should not have been used at all or referred to in the assessment. And Mr. Brennan’s push for providing an intelligence assessment to President Barack Obama before he left office meant that some intelligence officers had little opportunity to shape the document’s conclusions.