Samantha Dravis, Pruitt's former policy chief at EPA, told the investigators last week that Pruitt had made clear to her before and after he became EPA administrator that he would like the attorney general's job, held then and now by Jeff Sessions.
Pruitt tried to reverse efforts to cut emissions from coal plants, kill tougher fuel-efficiency standards for vehicles, and suspend stricter rules on runoff from farms - moves that elicited protests and, in several cases, legal challenges by environmentalists and some states.
Wheeler also had a number of other high-profile clients in the energy sector, according to the Washington Post, and many of these companies have an interest in the regulations Wheeler will now make decisions about as acting head of the EPA.
Andrew Wheeler, the EPA's current deputy administrator, will take over as the organisation's acting head on Monday, Mr Trump said.
Pruitt, as Oklahoma's attorney general before heading up the EPA, had sued the federal agency more than a dozen times on behalf of his oil-drilling state. "There are assumptions made that because the climate is warming that necessarily is a bad thing".
In a guest op-ed for the Orlando Sentinel from January 2017, when Trump first appointed Pruitt to lead the Environmental Protection Agency, Putnam wrote Pruitt "has the experience, understanding of the law and courage to get this out-of-control federal agency back on track".
Like President Trump, Wheeler is active on Twitter, although he has not tweeted since the news of his appointment was announced.
"Directly following the 2016 election, I voluntarily stopped any advocacy activity with EPA under the anticipation that I might be considered for a position at the Agency", Wheeler added.
"I have accepted the resignation of Scott Pruitt as the administrator of the Environmental Protection Agency", Trump said in a tweet.
Widespread disgust for Pruitt "should serve as a blaring red siren for the Trump administration", Udall said.
"The damage Scott Pruitt has done to the Agency will not easily be undone", he wrote. "Except when you're dealing with Trump, you're dealing with many issues of enormous consequences". The group Save the US EPA keeps a roster of endangered programs. The Washington Post reported that a longtime Pruitt friend and lobbyist helped arrange the trip and later registered as a foreign agent representing Morocco.
In April, the Government Accountability Office found Pruitt guilty of overspending $43,000 of congressionally allocated money on a soundproof booth installed in his office for privacy on personal calls. Pruitt said his staff never told him the cost.
Among other complaints, Pruitt is alleged to have pushed his aides to secure a $200,000 political-fundraising job for his wife; required staffers to perform non-official work; and insisted on traveling first-class or via private jet despite aides' objections.
Some of the tasks were outright weird, including a reported order to find and obtain a used Trump-brand mattress from a Trump hotel.