Special counsel Robert Mueller on Friday wrapped up his nearly two-year investigation into Donald Trump and Russia and sent his report to Attorney General William Barr.

No details of Mueller's findings have been released, but Barr said he may be able to brief congressional leaders on the report as soon as this weekend.

"I am reviewing this report and anticipate that I may be in a position to advise you of the Special Counsel's principal conclusions as soon as this weekend," Barr wrote in a letter on Friday to a group of lawmakers on the House and Senate judiciary committees.

Barr also said in his letter that he was required to inform congressional leaders if Mueller — whom Trump has relentlessly attacked as conducting a "witch hunt" — had proposed anything "inappropriate or unwarranted" as part of the probe.

"There were no such instances during the Special Counsel's investigation," the attorney general said, adding that he would determine how much of the report could eventually be released to Congress and the public.

Trump's outside lawyers, Rudy Giuliani and Jay Sekulow, issued a statement saying, "We're pleased that the Office of Special Counsel has delivered its report to the Attorney General pursuant to the regulations. Attorney General Barr will determine the appropriate next steps."

White House press secretary Sarah Sanders said, "We're pleased that the Office of Special Counsel has delivered its report to the Attorney General pursuant to the regulations. Attorney General Barr will determine the appropriate next steps.” In a tweet, she said that the White House had not seen the report.

The transmission of the report to Barr concludes an investigation that has resulted in the indictments of 34 people, infuriated the president and threw the administration into turmoil.

The Mueller report was delivered in person to the attorney general's office earlier Friday afternoon, before 4 p.m. At 4:30 pm, Deputy Attorney General Rod Rosenstein called Mueller and thanked him for his work.

Minutes later, Barr’s chief of staff called Emmett Flood, a White House lawyer, and read him the letter they were sending to Congress. The relevant committees in Congress all got the Mueller report at the same time, promptly at 5 pm.

The long-awaited end to the probe comes almost two years after Mueller was appointed by Rosenstein to investigate "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump" and "any matters that arose or may arise directly from the investigation."

To date, almost three dozen people and three companies have been criminally charged in the sprawling probe, including Trump's former national security adviser Michael Flynn; former campaign chairman Paul Manafort; former political adviser Roger Stone; former personal lawyer Michael Cohen; and numerous Russian nationals. There have been a number of guilty pleas and convictions — but none of the charges have directly accused anyone in Trump's orbit of conspiring with the Russian intelligence operation to help Trump get elected in 2016.

There will be no more indictments now that the probe is concluded, NBC News has learned.

It's unclear how detailed Mueller's report is, or when his conclusions may become public. According to Justice Department guidelines, his confidential report to the attorney general is supposed to explain "the prosecution or declination decisions reached by the special counsel."

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The attorney general is required to report Mueller's findings to Congress "with an outline of the actions and the reasons for them," the guidelines say, but it's unclear how long that may take.

As the Mueller investigation picked up steam and various Trump associates were charged, the president increasingly went on the offensive, blasting it as a "witch hunt" and "a hoax," calling Mueller's investigators "angry Democrats" and singling out some who'd worked on the case. He labeled Cohen "a rat" for cooperating with investigators.

Trump refused to sit for an interview with Mueller — his lawyers said they were concerned about a "perjury trap" — but he did submit written responses to Mueller's questions in November.

Mueller was appointed special counsel on May 17, 2017 — eight days after Trump fired James Comey as FBI director. Comey had been leading the investigation into Russian meddling and any possible Trump campaign involvement. The president initially said he'd canned Comey at the urging of Rosenstein and then-Attorney General Jeff Sessions, but later told NBC "Nightly News" anchor Lester Holt it was his decision, and cited his frustration with the Russia probe.

"And in fact when I decided to just do it, I said to myself, I said, you know, this Russia thing with Trump and Russia is a made-up story. It's an excuse by the Democrats for having lost an election that they should have won," Trump told Holt.

That fueled law-enforcement concerns that Trump was trying to obstruct the investigation — worries that were heightened a day after the firing, when he hosted two Russian diplomats in the Oval Office.

"I just fired the head of the FBI. He was crazy, a real nut job," Trump told them, according to The New York Times. "I faced great pressure because of Russia. That’s taken off."

Those and other actions taken by the president since the probe began led Mueller to investigate whether Trump was trying to obstruct justice in the case, sources have told NBC News.

The FBI probe into the campaign's alleged Russia ties started in July 2016 after a little-known Trump campaign foreign policy adviser, George Papadopoulos, told an Australian diplomat that the Russians had obtained thousands of emails that would embarrass Democratic nominee Hillary Clinton. The Australian government passed that information on to the FBI after hacked Democratic National Committee emails were posted online.

That wouldn't be the only hack. Russian cybercriminals targeted Clinton campaign chairman John Podesta's emails, U.S. intelligence officials found. They were released online just hours after the "Access Hollywood" scandal threatened to sink Trump's campaign.

Adding to investigators' suspicions was Trump's often-abrasive deference to Russian President Vladimir Putin, who he repeatedly praised as "tough" and "strong." He was also dismissive of U.S. findings that Russia was behind the cybercrimes, noting, "It also could be somebody sitting on their bed that weighs 400 pounds, OK?"

Trump initially denied that he and his campaign had anything to do with any Russians — claims that have since fallen apart.

Flynn had sat with Putin at a dinner in Moscow in 2015, and would be fired from his job as national security adviser for misleading Vice President Mike Pence about the content of his conversations with a Russian diplomat. Cohen and Trump associate Felix Sater were in talks during the campaign to develop a Trump Tower in Moscow, which would reportedly come complete with a multimillion-dollar apartment for Putin. The president's son Donald Trump Jr. set up a meeting at Trump Tower with a Russian entourage after they offered unspecified "dirt" on Clinton’s campaign courtesy of the Kremlin.

"If it's what you say I love it," Trump Jr. said in accepting the meeting, which would include Manafort and Trump's son-in-law and adviser, Jared Kushner.