Posters of Interior Secretary Ryan Zinke are plastered on street corners near the Interior Department offices in 2018. (Robert Miller/The Washington Post)

The Interior Department’s Office of Inspector General has opened an investigation into whether six of President Trump’s appointees have violated federal ethics rules by engaging with their former employers or clients on department-related business.

The new inquiry, which the office confirmed in an April 18 letter to the nonprofit Campaign Legal Center, is looking into senior Interior officials, including Assistant Secretary for Insular and International Affairs Doug Domenech, White House liaison Lori Mashburn, three top staffers at the Office of Intergovernmental and External Affairs, and the department’s former energy policy adviser. The Campaign Legal Center detailed the officials’ actions in a Feb. 20 letter to the inspector general’s office, suggesting a probe is warranted.

To avoid conflicts of interest, Trump signed an executive order days after taking office that requires appointees to recuse themselves from specific matters involving their former employers and clients for two years. The complaint, which cites reports in HuffPost and the Guardian as well as extensive public records, outlines how a half-dozen political appointees at Interior continued to discuss policy matters with organizations that had employed them in the past.

Delaney Marsco, ethics counsel at the Campaign Legal Center, said in a phone interview Tuesday that the inquiry shows that the president and his top deputies have failed allegedly to deliver on Trump’s 2016 campaign promise to “drain the swamp” in Washington.

“This is demonstrative of the failures at the very top of this administration to set an ethical tone,” Marsco said. “When people come to work for government, they’re supposed to work on behalf of the public. It’s a betrayal of the public trust when senior political appointees seem to give privileged access to their former employers or former clients.

“We hope this investigation will answer whether these officials are working on behalf of the American people or on behalf of the interests that used to pay their salary,” she added.

The Guardian reported in May 2018 that Domenech, the highest-ranking official named in the complaint, continued to interact with the conservative think tank that used to employ him before he joined Interior.

Domenech’s calendars indicated that he twice met with representatives from the Austin-based Texas Public Policy Foundation on an endangered species listing and a property dispute. The group had lawsuits pending with Interior over both issues and resolved the fight over private property near Texas’s Red River six months after the meeting with Domenech.

In a news release, the foundation described the November 2017 settlement with Interior as “a major win."

Benjamin Cassidy, a former lobbyist for the National Rifle Association, also has attracted significant attention for his past work on gun-related issues since joining the department as senior deputy director at its Office of Intergovernmental and External Affairs.

Calendars released by Interior show Cassidy participating in a December 2017 meeting regarding Trump’s decision to scale back two national monuments in Utah, even though Cassidy had lobbied Congress on a bill addressing the president’s ability to establish national monuments just months earlier. This activity, first reported by HuffPost, could violate the federal ethics pledge because Cassidy was prohibited from engaging in particular matters on which he had lobbied during the two years before joining the department.

Cassidy had also been in contact with a current NRA lobbyist, Susan Recce, about opening up Bureau of Land Management lands in Arizona and Utah to recreational shooting. Interior ultimately decided to allow recreational shooting in the Sonoran Desert National Monument, the option endorsed by the NRA.

After the decision was announced, the NRA published a post on its website quoting Recce as saying that “the BLM backed down from the closure alternative” as “a result of” the work her group and other gun rights advocates had done.

The center also alleges that Mashburn, a former associate director at the conservative Heritage Foundation, violated her ethics pledge by attending multiple private events held by her former employer.

Nancy DiPaolo, a spokeswoman for the inspector general, said in an email, “We have opened an investigation and are considering all the material presented by CLC, but because it is an active investigation, have no further comments.”

The move comes a week after the office launched a probe into whether Interior Secretary David Bernhardt violated federal conflict of interest rules by weighing in on policies that could affect the former clients he represented while working at the law firm Brownstein Hyatt Farber Schreck.

Bernhardt, who was confirmed earlier this month, has denied any wrongdoing and said he has cleared any action affecting his former employer or clients with the department’s ethics office.

Interior spokeswoman Faith Vander Voort said in an email that while the department does not typically comment on personnel matters, the secretary’s office “immediately consulted” with department ethics officials after receiving the center’s complaint in February.

“Ethics reviewed each matter and provided materials to the chief of staff, who has taken appropriate actions. All of these materials have been provided to the inspector general,” said Vander Voort, who declined to specify what actions the department had taken. “The department takes ethics issues seriously.”

The other two senior officials with the Office of Intergovernmental and External Affairs now under investigation are Todd Wynn and Timothy Williams. All of the officials named in the complaint continue to work at Interior, except for Vincent DeVito, who left his job as the department energy policy counselor in August to join an offshore oil drilling firm.