An 18-year veteran of the US Department of Interior has resigned in protest from the sprawling federal agency, denouncing “a deeply troubling assault on the rule of the law” that took place at the department under Donald Trump.

John Murdock, an attorney and self-described conservative Christian, began working for the agency’s legal office in 2002. In his resignation letter, first sent to his supervisor earlier this year and obtained exclusively by the Guardian and Public Domain, Murdock cites “glaring legal flaws” he witnessed within the agency during the course of the Trump administration, as well as concerns over offshore oil and gas drilling, the aggressive push to expand fossil fuels and the flouting of environmental regulation.

“We are headed in the wrong direction,” wrote Murdock, who officially stepped down on 30 September.

Murdock’s resignation and public comments come amid an exodus of longtime government workers and scientists within federal agencies, including at the interior department, which has lost thousands of employees ever since Elon Musk’s so-called “department of government efficiency” (Doge) began its controversial efforts to slash government employment last year.

“Sadly, I am but one of many civil servants who have departed the government under less than ideal circumstances in recent years. If my story helps others or in some small way assists in preserving the rule of law, I am willing to share it,” Murdock said.

In his resignation letter, Murdock said he grew especially concerned about the actions of a top official at the Bureau of Ocean Energy Management, or BOEM, an interior department agency that oversees offshore oil and gas drilling. His decision to resign in protest stemmed, in particular, from an incident that occurred last December, when Matt Giacona, the acting director of BOEM, appeared on stage at a gathering in New Orleans to announce the results of an oil and gas lease sale in the Gulf of Mexico. The lease sale received bids from 30 companies, including BP, Chevron and Shell, totaling more than $370m. During his speech, Giacona extolled the Trump administration’s pro-drilling agenda. He then claimed that the administration was “upholding the highest environmental standards”.

The lease sale in question, however, has been mired in legal controversy. Environmental groups and other critics contend the Trump administration explicitly bypassed numerous federal environmental laws, including provisions of the Endangered Species Act (ESA) and the National Environmental Policy Act (NEPA), while carrying out the sale. Known as BBG1, the lease sale was mandated by Trump’s One Big Beautiful Act. In a memo published shortly before the sale took place, BOEM’s Giacona, in coordination with the interior department’s legal office, determined that NEPA, a key provision of the ESA, and several other environmental safeguards “are not applicable at the lease sale stage of” BBG1 and similar sales. Environmental groups quickly filed a lawsuit challenging the legality of Giacona’s actions.

“The Trump administration has no basis for exempting these massive drilling lease sales from the bedrock laws that Congress passed decades ago after destructive oil spills off our coasts,” said George Torgun, a senior attorney at Earthjustice, one of the environmental groups involved in the ongoing lawsuits against BBG1 and similar lease sales. “This illegal action is a payback to oil and gas drillers. Gulf communities – and the marine ecosystems that we all depend on – will pay the price.”

The administration pushed back on such arguments in a legal filing earlier this year. Because the lease sale was mandated by Congress, the agency says it did not have “sufficient discretion to alter the scope or terms of the lease sale in ways that could affect environmental outcomes”.

When Murdock heard Giacona claim that the administration was upholding the highest environmental standards, he decided he needed to resign. “I felt an ethical obligation to highlight the glaring legal flaws I saw,” he wrote of his actions leading up to his resignation.

An interior department spokesperson, in a statement, said that “one former employee’s personal disagreement with the Department’s policies does not transform lawful policy decisions into an ‘assault on the rule of law,’ nor does his inflammatory rhetoric make his opinion accurate. The Department stands firmly behind its work to implement President Trump’s agenda consistent with federal law.”

Among other troubling policies Murdock witnessed while at DOI was the Trump administration’s decision this year to exempt oil and gas operations in the Gulf of Mexico from the Endangered Species Act. “Whether technically legal or not, it is a repugnant policy that I in no way support,” Murdock wrote in his letter. “That repugnancy is especially true with one of the most imperiled species of marine mammals, the Rice’s whale, calling the Gulf home.”

Murdock’s work at the interior department included efforts “to help clean up a mess that BP had made” during the Deepwater Horizon oil spill. “I do not want to now be part of creating an even greater disaster,” he wrote.

Murdock also protested the administration’s continuing political attack on offshore wind energy. Under the current Trump administration’s energy policies, he wrote, an “all of the above” energy strategy has morphed into “one of the above”, solely focused on fossil fuels. “The recent decisions to shutter nearly complete offshore wind projects and to pay Total Energies hundreds of millions of dollars to renounce wind leases are an assault on logic and the American taxpayer,” he added, referencing the administration’s announcement earlier this year that it would reimburse a French energy company nearly $1bn to forgo plans to build wind farms off the US east coast.

He denounced too “the weekly propaganda video, Inside Interior, sent to every employee’s inbox. Though some version of a self-congratulatory update has long existed, the change in tone to something approaching cultish worship has been jarring.” A recent Inside Interior video, for instance, described “the fearless leadership of President Donald J. Trump, who reminds us everyday what true patriotism looks like as he works tirelessly to make America great again”.

In resigning, Murdock, who now lives in Texas, acknowledged that he will likely pay a personal price. “I walk away from the stable salary and benefits of a government job and my retirement likely becomes less lucrative and more complicated,” he wrote. “Yet, I will gain much – including a deeper admiration for those who have made much greater sacrifices – and I will preserve my integrity.”

BOEM’s Giacona, meanwhile, departed the agency in September to go work for oil services giant Halliburton.

The Guardian and Public Domain reached out to Halliburton seeking comment from Giacona, but did not receive a response.

• This article was co-published with Public Domain, an investigative newsroom covering public lands, the interior department and wildlife.