Confidential security assessments in the Environmental Protection Agency show no evidence of specific, credible, physical threats against administrator Scott Pruitt, despite claims that an “unprecedented” number of death threats justify his outsized security spending, according to a review by Senate Democrats.

Democrats on the Senate environment and public works committee wrote in a letter on Tuesday that they have reviewed security assessments describing 16 purported threats against Pruitt. They include public protests, criticism of Pruitt’s policies and other activities protected by the first amendment.

The letter from Tom Carper of Delaware and Sheldon Whitehouse of Rhode Island urged the committee’s Republican majority to begin oversight hearings into Pruitt’s unusual security precautions.

The committee chairman, John Barrasso, quickly rebuffed that request, saying no hearing on Pruitt’s security procedures will be held. Barrasso, a Wyoming Republican, lashed out at his Democratic colleagues, accusing Carper and Whitehouse of selectively quoting the documents and improperly disclosing “law enforcement sensitive information” without the approval of the EPA or the secret service.

The Associated Press reported on Friday that the EPA has spent about $3m on Pruitt’s security measures, which included flying first-class and using a full-time security detail of 20 armed officers.

EPA spokesman Jahan Wilcox said last week there had been an “unprecedented” number of death threats against Pruitt and his family. Wilcox doubled down on that assertion on Tuesday, but did not immediately respond to a request from AP to release details of the specific incidents to which he was referring.

Donald Trump defended Pruitt in a tweet Saturday night, downplaying the ethical questions swirling around his embattled EPA chief. He added that Pruitt’s security spending was “somewhat more” than prior EPA chiefs, but said that Pruitt had received death threats “because of his bold actions at EPA”.

The Democrats said they found no records describing specific, credible threats against Pruitt. An internal EPA document recounted such threat as attempts by protesters to disrupt a speech and a post card sent to Pruitt that said: “CLIMATE CHANGE IS REAL!!! We are watching you.”

The Democrats also obtained an assessment, dated 14 February, from EPA’s Office of Homeland Security Intelligence that concluded “EPA intelligence has not identified any specific, credible, direct threat to the EPA administrator”.

The internal review said an earlier threat assessment by Pruitt’s security team “does not employ sound analysis or articulate relevant ‘threat specific’ information appropriate to draw any resource or level of threat conclusions regarding the protection posture for the administrator.”

Politico first reported on Tuesday that the career EPA security staffer who wrote the February memo, Mario Caraballo, was fired shortly after the Democrats’ account of it became public.

The Democratic senators called the timing of Caraballo’s firing “deeply troubling”.

“This development underscores the need for the environment and public works committee to conduct effective oversight of the EPA to answer the serious questions that have come to light in recent days concerning management and ethical conduct by the administrator and his staff,” Carper and Whitehouse said in a joint statement.

The AP reported on Friday that Pruitt’s concern with his safety came at a steep cost to taxpayers as his swollen security detail blew through overtime budgets and at times diverted officers away from investigating environmental crimes. Pruitt’s 20-member full-time detail is more than three times the size of his predecessor’s part-time security contingent.

The EPA’s inspector general is currently auditing Pruitt’s security spending – one of at least five ongoing investigations by the agency’s watchdog into spending and ethics issues surrounding the administrator. A GOP-led House oversight committee is also investigating Pruitt’s travel spending and the administrator’s bargain-priced rental of a $50-a-night Capitol Hill condo tied to a fossil fuels lobbyist.

Shortly after arriving in Washington, Pruitt, 49, demoted the career staff member heading his security detail and replaced him with EPA senior special agent Pasquale “Nino” Perrotta, a former Secret Service agent who operates a private security company.

Perrotta also signed off on new procedures that let Pruitt fly first-class on commercial airliners.