WASHINGTON, May 20 (Xinhua) -- U.S. President Donald Trump said Sunday that he will demand that the Department of Justice (DOJ) investigate whether the Federal Bureau of Investigation (FBI) "infiltrated" his 2016 campaign.

"I hereby demand, and will do so officially tomorrow, that the Department of Justice look into whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes -- and if any such demands or requests were made by people within the Obama Administration!" Trump tweeted Sunday.

The message, which came amid a flurry of tweets the president unleashed Sunday attacking the Russia probe led by special counsel Robert Mueller, was an apparent reference to a New York Times story.

The story reported Friday that an FBI informant met with a number of Trump campaign associates in the summer of 2016, as part of the agency's investigation into the alleged Russian interference with the presidential election that year.

Representative Devin Nunes, the Republican chairman of the House Intelligence Committee, has asked the DOJ to turn over documents disclosing why the FBI launched an investigation into the Trump campaign.

It is "an absolute red line" if the FBI ran a spy ring inside Trump's campaign, Nunes said.

Democrats said that the president's threatened investigation would amount to "an abuse of power."

Representative Adam Schiff, the ranking Democrat on Nunes' panel, tweeted Sunday that "Trump's claim of an embedded 'spy' is nonsense."

"His 'demand' DOJ investigate something they know to be untrue is an abuse of power, and an effort to distract from his growing legal problems," Schiff added.

Rudy Giuliani, Trump's lawyer, said that the president would be "walking into a trap" if he agreed to be interviewed by Mueller's team without clarification on the role of the informant.

"What we intend to do is premise it on, 'If you want an interview, we need an answer to this,'" Giuliani told The Wall Street Journal in an interview published on Sunday.

Giuliani also said Sunday that Mueller plans to finish his probe by Sept. 1, a claim that the special counsel's team has not commented.

Hours after Trump's tweet, a DOJ spokeswoman said the department has asked its inspector general to expand a probe launched in March.

Deputy Attorney General Rod Rosenstein said in a statement: "If anyone did infiltrate or surveil participants in a presidential campaign for inappropriate purposes, we need to know about it and take appropriate action."

The informant, identified by numerous news outlets as Cambridge professor Stefan Halper, met with campaign aides Carter Page, Sam Clovis and George Papadopoulos in 2016.

Page and Clovis testified to grand juries, and Papadopolous pleaded guilty last year to lying to the FBI about his contacts with Russians and is cooperating in the Mueller probe.

The FBI had been monitoring Page since 2013 when they learned Russian agents were trying to recruit him, according to reports.

The Mueller probe reached its one-year mark last week.

Trump, who has repeatedly called the Russia probe a "hoax" or "witch hunt", has ratcheted up pressure on it in the past months, amid a reshuffling of his legal team.

Mueller is mandated to investigate the alleged Russian interference in the 2016 U.S. presidential election and any potential collusion between the Trump campaign and Moscow, among other matters that may arise from the investigation.

Over the past year, Mueller's team has brought charges against 19 individuals, including several Trump campaign associates, and three Russian entities, notched five guilty pleas and seen one person sentenced.

A number of those charges were related to Russia's alleged interference in the 2016 election, but none of them has been extended to collusion allegations.

Russia has denied interference in the election campaign.