The oil industry's most powerful lobbying group met on March 23 with President Trump's interior secretary at the Trump International Hotel in Washington, DC. It also happened to be the same day the administration killed a rule that oil companies opposed.

The location of the meeting is raising eyebrows and ethical questions. The Trump International Hotel, situated just blocks from the White House, is ground zero for companies and foreign leaders who may be trying to cozy up to the president by using his properties, critics and ethics experts fear.

"It creates the appearance they are currying favor" by staying at a Trump hotel, said Lawrence Noble, general counsel at the Campaign Legal Center, a nonprofit, nonpartisan watchdog.

Noble, a CNN contributor, said while the meeting may not violate specific ethics rules, it shows that companies have discovered a "not-so-subtle way of showing support for the president."

Interior Secretary Ryan Zinke addressed the American Petroleum Institute's (API) board of directors on that day at the Trump International Hotel, according to Zinke's recently-released schedule.

Zinke, a strong advocate of the oil industry, spoke for 10 minutes, and then held a brief question-and-answer session, the Interior Department confirmed in a statement to CNNMoney.

That same day, the Interior Department announced plans to get rid of an Obama-era rule toughening standards on how much fossil fuel companies owe the government for drilling and mining on federal land. The energy industry had fought the rule. The oil industry group had even filed a lawsuit against it in December 2016.

The very next day, on March 24, the API put out a statement saying it was "pleased" by the Interior Department's decision to get rid of the rule's "substantial burdens."

Related: Trump business launching mid-scale hotel chain

The Interior Department defended Zinke's appearance at the API event.

"Like many secretaries before him, the Secretary was invited to speak at API's meeting and he accepted the invitation. There is nothing unusual about a secretary speaking to stakeholders," Heather Swift, a spokesperson for the department, said in a statement.

Swift said Zinke spoke about his "goals for the Department of the Interior and American energy."

The Interior Department's ethics office said it had "thoroughly vetted" Zinke's API meeting. "We found that it presented no ethics violation or conflict of interest," the ethics office said.

Noble, the ethics expert, agrees that meeting with an industry group "in and of itself is not unusual" as long as Zinke didn't insist the gathering take place at a Trump hotel. There's no evidence that Zinke picked the location of the API meeting.

It's not clear how much the API spent on holding the meeting at the Trump International Hotel. Events at the hotel likely cost at least $100,000, The Washington Post has previously reported.

Neither the API nor the Trump Organization responded to requests for comment.

Zinke's schedule doesn't indicate who attended the meeting with API, which is chaired by ConocoPhillips (COP) CEO Ryan Lance.

Related: Trump plan for hotel profits blasted by top Democrat

The Trump International Hotel, which opened last September on the grounds of a renovated post office, has been a lightning rod for controversy. The Trump Organization rents space for the hotel from the General Services Administration, an agency of the Untied States government.

As president, Trump oversees the GSA, which makes him effectively both landlord and tenant.

Critics have argued the hotel violates the lease terms because there is a clause saying no government official can be a party to the 60-year lease that was signed in 2013.

In March, the federal government ruled that the hotel is not in violation of its lease. The GSA cited Trump's decision to transfer control of his vast business empire to his sons and a Trump Organization executive.

However, Trump is still the ultimate beneficiary of the success of the company and the hotel.

Noble said there's an easy way to resolve concerns about such conflicts involving Trump's hotel.

"Just decide you won't do any government business at the president's hotel. Set a rule," he said.

--CNNMoney's Jill Disis and Julia Horowitz contributed to this report.