WASHINGTON -- 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 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 Sens. 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.

Committee Chairman John Barrasso quickly rebuffed that request, saying no hearing on Pruitt's security procedures will be held. The 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 EPA or the Secret Service.

The Associated Press reported Friday that EPA has spent about $3 million 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" amount of death threats against Pruitt and his family. Wilcox doubled down on that assertion Tuesday, but did not immediately respond to a request from AP to release details of the specific incidents to which he was referring.

President Donald Trump , downplaying the ethical questions swirling around his embattled EPA chief. He added that Pruitt's security spending "somewhat more" than prior EPA chiefs, but said 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 a Feb. 14 assessment 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 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. He was the deputy associate administrator of EPA's Office of Homeland Security.

Asked about Caraballo's dismissal, EPA suggested the timing was a coincidence.

"I am not aware of any connection between the personnel matter and the document mentioned in media reports," said Donna Vizian, the principal deputy assistant administrator for administration, according to a statement released by Pruitt's staff.

The Democratic senators called 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.

Caraballo could not be reached for comment Tuesday.

The AP reported 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.

EPA's inspector general is currently auditing Pruitt's security spending -- one of at least five ongoing probes 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 oversaw a rapid expansion of the EPA chief's security detail to accommodate guarding him day and night, even on family vacations and when Pruitt was home in Oklahoma.

Perrotta also signed off on new procedures that let Pruitt fly first-class on commercial airliners, with the security chief typically sitting next to him with other security staff farther back in the plane. Pruitt's premium status gave him and his security chief access to VIP airport lounges.

Pruitt has said his use of first-class airfare was initiated following unpleasant interactions with other travellers. In one incident, someone yelled profanity as he walked through the airport.

A nationwide search of state and federal court records by AP found no case where anyone has been arrested or charged with making violent threats against Pruitt.

In November, BuzzFeed News investigative reporter Jason Leopold filed a public records request with EPA for copies of government records about death threats to Pruitt. Leopold told AP this week that an EPA official who responded to his request told him verbally that after checks with the agency's general counsel and inspector general's office that no such records existed.