Udall, describing Pruitt's management of the agency as "disastrous", again called on Pruitt to resign.
The EPA's Inspector General reported this week that Pruitt did ask for the unusual level of security as soon as he was confirmed, and that the cost of such security has run up a bill of $3 million dollars.
The request for the email investigation came from Democratic Sens.
Udall also signed onto a letter last week with other Democratic lawmakers questioning EPA's expenses in refurbishing Pruitt's office, including its installation of a secure phone booth. Pruitt says, "I don't recall that happening". He was speaking during a hearing that was meant to focus on the EPA's 2019 budget but that centered on questions about his conduct - including an allegation he had sought to be transported through traffic with flashing lights and sirens.
Pruitt's security chief, Pasquale "Nino" Perrotta, announced his early retirement from EPA earlier this month after questions arose about whether he improperly recommended a business partner for a federal contract and outside work he performed as a private investigator for a tabloid newspaper.
According to EPA's inspector general, Pruitt had himself asked for the security, which was given to him.
Republican Sen. Lisa Murkowski of Alaska, the subcommittee chairwoman, has told reporters that she was concerned about the allegations surrounding Pruitt, but that it was up to the Trump administration to decide his future.
"Unfortunately, I am concerned that numerous important policy efforts you are engaged in are being overshadowed", she said. She signaled she'll have questions for Pruitt on both policy and ethics. Pruitt's staff has previously said the enhanced protection, which far exceeds that afforded to past EPA administrators, was triggered by death threats. The senior senator called "silly" Pruitt's claims he needed to fly in first class after unpleasant interactions with other travelers.
Udall said he had asked the nonpartisan Government Accountability Office to investigate the EPA's use of social media to send a politically driven Tweet.
Senator Chuck Grassley, for example, said on Tuesday he would join lawmakers calling for Pruitt's ouster unless the EPA chief curbs the agency's use of waivers exempting refineries from their obligation to mix ethanol into fuel.
The study would show that the chemicals - known as PFOA and PFOS - are risky to humans in much smaller quantities than the Environmental Protection Agency has previously said, Politico reported this week.
"Then that's a gift, that's a violation of federal law", Udall said, noting that subordinates of federal officials are prohibited from providing free services to their bosses. Former EPA officials have told The Associated Press that Pruitt made the change after Perrotta's predecessor refused to use lights and sirens in non-emergency situations, such as when the administrator was running late for dinner reservations or going to the airport. In the tweet, the agency appeared to mock Democrats after the long-awaited confirmation of former coal lobbyist Andrew Wheeler as Pruitt's top deputy.