FARGO, N.D. >> President Donald Trump declared today the U.S. Justice Department should investigate and unmask the author of a bitingly critical New York Times opinion piece purportedly written by a member of an administration “resistance” movement straining to thwart his most dangerous impulses.

Trump cited “national security” as the reason for such an extraordinary probe, and he called on Attorney General Jeff Sessions to get it going. He also said he was exploring bringing legal action against the newspaper over publication of the essay two days earlier.

“Jeff should be investigating who the author of that piece was because I really believe it’s national security,” Trump said. If the person has a high-level security clearance, he said, “I don’t want him in those meetings.”

A White House official later said Trump’s call for a government investigation was an expression of his frustration with the essay, not an order for federal prosecutors to take action.

Though the article was strongly critical of Trump, no classified information appears to have been revealed by the author or leaked to the newspaper, which would be a crucial bar to clear before a leak investigation could be contemplated.

“The department does not confirm or deny investigations,” said Sarah Isgur Flores, a spokeswoman.

Trump’s call is the latest test of the independence of his Justice Department, which is supposed to make investigative and charging decisions without political interference from the White House.

It also reflects an expansive view of White House authority that cuts to the heart of special counsel Robert Mueller’s investigation.

Trump’s lawyers and supporters have argued that as president he is empowered not only to hire and fire whomever he chooses but that he can also inject himself into law enforcement matters.

Former FBI Director James Comey has said Trump asked him to go easy on former national security adviser Michael Flynn, an allegation that Mueller is scrutinizing. Trump’s lawyers have said that even if that happened as Comey described — they say it didn’t — it can’t be a crime because the president has the constitutional authority to involve himself in the activities of the Justice Department.

Trump’s call for an investigation came a day after his top lieutenants stepped forward to repudiate the op-ed in a show of support for their incensed boss, who ordered aides to unmask the writer. The denials continued today, when U.N. Ambassador Nikki Haley published an opinion piece of her own in The Washington Post titled, “When I challenge the president, I do it directly. My anonymous colleague should have, too.”

Democrats were quick to condemn the president’s call for a federal investigation.

Sen. Chris Coons of Delaware said, “President Trump continues to show a troubling trend in which he views the Department of Justice as the private legal department of the Trump organization rather than an entity that is focused on respecting the Constitution and enforcing our laws.”

Minnesota Sen. Amy Klobuchar said she wished Trump “would put the same vigor into getting to the bottom of what Russia has been doing to our country in the elections.”

But Rudy Giuliani, the president’s attorney, suggested that it “would be appropriate” for Trump to ask for a formal investigation into the identity of the op-ed author.

“Let’s assume it’s a person with a security clearance. If they feel writing this is appropriate, maybe they feel it would be appropriate to disclose national security secrets, too. That person should be found out and stopped,” Giuliani said.

Kentucky Sen. Rand Paul, a key ally of Trump’s, called for the president to order those suspected of being the author to undergo lie-detector tests.

“People are suggesting it,” Trump said today, steering clear of explicitly endorsing the proposal. “Eventually the name of this sick person will come out.”

In an interview Thursday with Fox News, Trump said the author “may not be a Republican, it may not be a conservative, it may be a deep state person who has been there for a long time.”

The anonymous author, claiming to be part of the resistance “working diligently from within” the administration, wrote that, “Many Trump appointees have vowed to do what we can to preserve our democratic institutions while thwarting Mr. Trump’s more misguided impulses until he is out of office.”

“It may be cold comfort in this chaotic era, but Americans should know that there are adults in the room,” the author continued. “We fully recognize what is happening. And we are trying to do what’s right even when Donald Trump won’t.”