Rep. Trey Gowdy, R-S.C., on Friday demanded interviews with five top aides to embattled Environmental Protection Agency Administrator Scott Pruitt, escalating a probe his committee is conducting over Pruitt's spending and ethics.

The chairman of the Oversight and Government Reform Committee wrote a letter to Pruitt asking him to make available five aides for transcribed interviews: Pasquale “Nino” Perrotta, the head of Pruitt's security detail; chief of staff Ryan Jackson; Millan Hupp and Sarah Greenwalt, long-time advisers to Pruitt who were approved for large raises; and Kevin Chmielewski, a former deputy chief of staff.

Perrota is under scrutiny for approving waivers from federal rules for Pruitt to travel first class for security reasons.

Pruitt is the first EPA administrator to have a 24/7 security detail, prompted by multiple death threats he claims to have received. The Associated Press last week reported the EPA has spent about $3 million on his security, including travel and overtime pay for Pruitt’s detail, much of it approved by Perrota.

The EPA inspector general is investigating Perrotta for directing an EPA contract to a business partner who conducted a search for surveillance devices in Pruitt's office, according to the New York Times.

Chmielewski is a former Trump campaign aide and deputy chief of staff to Pruitt who told Democratic lawmakers this week that he was pushed out of the agency after he refused to approve first-class travel retroactively for another EPA aide.

According to Democratic lawmakers, Chmielewski described to them concerns he had with Pruitt’s spending related to his personal security, including purchases of bulletproof vests and weapons, biometric locks, and the security sweep of his office.

Chmielewski, the Democrats said, also disputed Pruitt’s claim that he didn’t know about massive raises the EPA granted to Hupp and Greenwalt, two aides who worked with Pruitt in Oklahoma. The agency granted the raises after the White House refused to approve them.

Republican staff of the Oversight and Government Reform Committee on Thursday met with Chmielewski, who is being treated as a whistle-blower, to corroborate his account of working for Pruitt.

Gowdy's committee is also investigating Pruitt for signing a $50-per-night lease agreement last year to live in the bedroom of a condo owned by the wife of energy lobbyist J. Steven Hart.

The Friday letter to Pruitt is the second Gowdy has sent to him this week.

On Wednesday, Gowdy said the EPA has failed to provide all documents he requested nearly two months ago related to Pruitt’s frequent use of first-class flights.

The EPA has provided the committee some documents, including those showing that Pruitt spent more than $105,000 in first class travel in his first year.

Gowdy gave the EPA an April 27 deadline to respond to his new interview requests.