President Trump on Friday repeatedly called on the Department of Justice and FBI to investigate his Democratic political opponents, a breach of the traditional executive branch boundaries designed to prevent the criminal justice system from becoming politicized.

Trump urged federal law enforcement to "do what is right and proper" by launching criminal probes of former presidential rival Hillary Clinton and her party — a surprising use of his bully pulpit considering he acknowledged a day earlier that presidents are not supposed to intervene in such decisions.

In a flurry of accusatory morning tweets, Trump claimed there was mounting public pressure for new Clinton probes, including over her campaign's joint fundraising agreement with the Democratic National Committee that effectively gave her some control over the party's finances, strategy and staffing before the primaries began.

Trump invoked Sen. Elizabeth Warren (D-Mass.), who had said that she believed the Democratic primaries were rigged in Clinton's favor based on details of the arrangement in a new book by former DNC interim chair Donna Brazile. Using his pejorative nickname for Warren, Trump tweeted: "Pocahontas just stated that the Democrats, lead [sic] by the legendary Crooked Hillary Clinton, rigged the Primaries! Lets [sic] go FBI & Justice Dept."

[Clinton campaign, DNC paid for research that led to Russia dossier]

Trump also called for probing the deleted emails from Clinton's private server while she was secretary of state, as well as the sale of a uranium company to Russia and the international business of Democratic super-lobbyist Tony Podesta, the brother of John D. Podesta, who served as Clinton's campaign chairman.

"People are angry," Trump wrote in one tweet. "At some point the Justice Department, and the FBI, must do what is right and proper. The American public deserves it!"

Trump amplified his message later Friday morning, as he spoke to reporters on the South Lawn of the White House before he departed for a 12-day trip to Asia.

"I'm really not involved with the Justice Department," Trump said. "I'd like to let it run itself. But honestly, they should be looking at the Democrats . . . And a lot of people are disappointed in the Justice Department, including me."

Trump has long been irritated, and at times outright angry, with Attorney General Jeff Sessions for refusing to prosecute Clinton and for not better protecting him from special counsel Robert S. Mueller III's wide-ranging probe into Russian interference in the 2016 election, the president's advisers have said.

Trump made his displeasure clear in a Thursday radio interview on "The Larry O'Connor Show."

"You know, the saddest thing is, because I am the president of the United States, I am not supposed to be involved with the Justice Department. I'm not supposed to be involved with the FBI," Trump said. "I'm not supposed to be doing the kind of things I would love to be doing and I am very frustrated by it."

The president said it was "very discouraging to me" that the Justice Department and FBI were not "going after Hillary Clinton." He added, "Hopefully they are doing something and at some point, maybe we are going to all have it out."

The White House offered no explanation for why Trump publicly pressured the Justice Department on Friday. A Justice Department spokesman also declined to comment.

Senior officials at the White House and some key Republican lawmakers have raised concerns in recent days about the level of access the Justice Department has been providing to congressional investigators to review materials.

Some lawmakers have sought more information than the department has provided related to a dossier that contained lurid allegations about Trump's alleged ties to Russia, in addition to material related to the uranium deal, according to administration and congressional officials.

A White House official said Trump wants the Justice Department, as a general policy, to be transparent and provide Congress with the information it is requesting. A Justice Department official said the White House overtures were not considered inappropriate because they were about the agency's compliance with congressional oversight. Both officials spoke on the condition of anonymity because they were not authorized to discuss the matter on the record.

Friday's public pressure marks the latest attempt by Trump to use his presidential megaphone to direct the criminal justice process.

Trump delivered off-the-cuff remarks this week recommending punishment for Sayfullo Saipov, the suspect accused of killing eight people with a rental truck in New York City. He at first said he was considering sending Saipov to the military prison at Guantanamo Bay, Cuba, but then reversed course and advocated a civilian trial in federal court and the death penalty for the terrorism suspect he called "an animal."

The president's comments complicated the work of FBI agents and federal prosecutors as they were investigating the attack and preparing criminal charges.

[Trump speaks, but prosecutors may not listen]

The Justice Department is a part of the executive branch; the attorney general is nominated by the president, as is the FBI director. So it is normal for the White House to direct the department and bureau on broad policy goals.

But unlike other executive branch agencies, the Justice Department traditionally enjoys a measure of independence, especially when it comes to individual criminal investigations. Government lawyers have long sought to enforce a clear line preventing White House officials from influencing specific investigations or prosecutions to ensure their work is not politicized.

Trump has departed from tradition when it comes to the Justice Department in other ways as well, including by talking with some candidates for some U.S. attorney jobs. Although they are presidential appointees, U.S. attorney candidates have not traditionally had personal interviews with the president before they were selected.

Sen. Bob Corker (R-Tenn.), who is not running for reelection and has become an outspoken critic of Trump's, issued a statement saying that the justice system should be "independent and free of political interference."

"President Trump's pressuring of the Justice Department and FBI to pursue cases against his adversaries and calling for punishment before trials take place are totally inappropriate and not only undermine our justice system but erode the American people's confidence in our institutions," Corker said.

Matt Axelrod, who served as the principal associate deputy attorney general interacting with the White House during the Obama administration, said Trump's comments were "a very troubling and shocking departure from the way things are supposed to work and have worked historically through both Democratic and Republican administrations."

But former attorney general William P. Barr, who served under former Republican president George H.W. Bush, said it would not be automatically inappropriate for a president to ask for possible wrongdoing to be investigated.

"The president is the chief executive and, if he believes there's an area that requires an investigation, there's nothing on its face wrong with that, there's nothing per se wrong about that," Barr said.

"I don't think all this stuff about throwing [Clinton] in jail or jumping to the conclusion that she should be prosecuted is appropriate," Barr added, "but I do think that there are things that should be investigated that haven't been investigated."

The president directing a particular investigation — especially of a former political rival — would be viewed by most officials in law enforcement as improper. This, though, is not the first time Trump has suggested putting Clinton in law enforcement's crosshairs.

During the campaign, Trump repeatedly led rally crowds in chants of "Lock her up!" that were aimed at Clinton. And during a presidential debate, Trump told Clinton he would "instruct my attorney general to get a special prosecutor to look into your situation, because there has never been so many lies, so much deception."

Trump's comment sparked a torrent of criticism, including from former attorney general Michael Mukasey, who worked under former Republican president George W. Bush and was a vocal Clinton critic.

"It would be like a banana republic," Mukasey, who could not be reached for comment Friday, said at the time. "Putting political opponents in jail for offenses committed in a political setting, even if they are criminal offenses — and they very well may be — is something that we don't do here."

Devlin Barrett, Spencer S. Hsu and Julie Tate contributed to this report.