WASHINGTON – Special counsel Robert Mueller completed his investigation into Russian interference in the 2016 election on Friday, delivering a report that signals the end of the long-running inquiry that loomed over the first two years of Donald Trump's presidency and saw a half-dozen of his top aides convicted of federal crimes.

Attorney General William Barr offered no clue what the investigation concluded, but the Justice Department said a summary of its findings could become public as soon as Saturday.

Mueller did not recommend any additional indictments, according to a Justice Department official who is not authorized to speak publicly.

Barr said in a letter to Congress Friday afternoon that Mueller had submitted his final, confidential report, and that he could alert lawmakers in the coming days to its findings. Findings of the report, described as "comprehensive" by Justice Department spokeswoman Kerri Kupec, will likely be made public at the same time.

"I write to notify you ... that special counsel Robert S. Mueller III has concluded his investigation of Russian interference in the 2016 election and related matters," Barr wrote in a letter to the top lawmakers on the House and Senate judiciary committees.

Barr told lawmakers he is "reviewing the 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."

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White House spokeswoman Sarah Sanders said the White House had not received Mueller's report or been briefed on its findings.

Reaction on Capitol Hill was largely circumspect, with Democrats and Republicans calling for the swift release of Mueller's findings. House Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer called for the full report to be made public.

Sen. Chuck Grassley, R-Iowa, was more tart. He praised Mueller for his “integrity and thoughtfulness” and called on Barr to “provide Congress and the American people with the findings to finally put an end to the speculation and innuendo that has loomed over this administration since its earliest days."

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The delivery of Mueller's report caps a remarkable investigation launched in secret months before Trump was elected president, when the U.S. government began gathering evidence that a Russian intelligence service had hacked into Democratic political organizations and released troves of stolen documents, in part to help Trump win the White House. At the same time, the FBI found clues that aides to Trump's campaign had tried to coordinate with the Russian effort.

In the nearly two years since Mueller was installed to oversee that work, the investigation has resulted in charges against more than two dozen Russian operatives, and a succession of aides and advisers to Trump's campaign, including its former chairman. In doing so, prosecutors mapped the details of a Russian operation to sway the election, and a campaign eager to reap the benefit of those efforts.

The now-completed probe has not resulted in charges that anyone associated with Trump coordinated with the Russians.

The delivery of Mueller's report means his investigation, which included an inquiry into whether Trump tried to obstruct justice, ends without investigators having interviewed the president. Trump's lead lawyer, Rudy Giuliani, said Trump had submitted written answers to questions from the special counsel, but had not submitted to an interview.

Barr told lawmakers Friday he was "committed to as much transparency as possible," and would consult with Mueller and other Justice Department officials to determine how much of the report could become public.

A security officer from Mueller's office delivered the report to Deputy Attorney General Rod Rosenstein, who turned it over to Barr. The Justice Department notified the White House just before 5 p.m., and Rosenstein called Mueller to express his "appreciation" for his work.

On Friday evening, Barr remained in his fifth-floor office reviewing Mueller's findings and preparing to assemble a written summary, Kupec said.

A spokesman for Mueller, Peter Carr, said the special counsel would be "concluding his service in the coming days." He said a small staff would remain to close down the office.

Barr said neither he nor his predecessors had overruled any of Mueller's investigative actions.

Trump has dismissed the probe as a "hoax" and a "witch hunt" led by political opponents in the Justice Department and the FBI. Among its most immediate consequences was a remarkable rift between the president and his appointees inside the Justice Department. In particular, former Attorney General Jeff Sessions, one of Trump's earliest supporters who became a frequent target after he recused himself from overseeing the Russia investigation.

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Mueller took over the investigation in May 2017. Since then, his prosecutors indicted dozens of defendants on hundreds of charges. Among the most high profile: Michael Cohen, Trump's longtime personal lawyer and fixer, and Michael Flynn, Trump's former National Security Adviser and a member of his campaign staff. His onetime campaign manager, Paul Manafort, also was convicted in one case and pleaded guilty in another.

Barr must now determine how much of Mueller's report to send to Congress, and must also notify lawmakers of whether he or his predecessors rejected any of the steps Mueller proposed taking in his investigation. It will be up to lawmakers to determine what to investigate, particularly where criminal charges weren’t filed. Some Democrats in the House have called for Trump’s impeachment.

Why do we have a Mueller investigation?

In May 2017, Rosenstein appointed Mueller to investigate possible cooperation between Trump’s campaign and Russians seeking to influence the election. By that point, Flynn had resigned for misleading others in the administration about meeting with Russians during the transition.

Days before the appointment, Trump fired FBI Director James Comey amid an investigation of Trump campaign ties to Russia. And Trump met with Russian Foreign Minister Sergei Lavrov and Ambassador Sergey Kislyak in the Oval Office.

Mueller took over an FBI counter-intelligence investigation that began in the summer of 2016 after agents learned that a campaign aide, George Papadopoulos, had boasted to an Australian diplomat that Russia had obtained damaging information about Hillary Clinton. The claim took on new significance that summer after investigators determined that Russian intelligence had hacked the Democratic National Committee and stolen troves of email.

Rosenstein said he hadn’t determined that any laws were broken, but that the unique circumstances required an investigation independent of the normal chain of command.

Charges, convictions in investigation

Among the dozens caught up in the Mueller investigation or related inquiries by federal prosecutors in New York and Washington are close Trump associates, political operatives and advisers and Russian nationals and entities. Many cases are pending, some men are awaiting sentencing and at least two have served their sentences.

Those charged include:

Roger Stone, a longtime Trump associate and political consultant, who was indicted in January on seven charges related to lying to investigators about efforts by top Trump campaign aides to learn about emails the Russian government had stolen from his political rivals.

Michael Flynn, a former Army lieutenant general who served as Trump’s national security adviser and worked on the campaign, pleaded guilty in December 2017 to lying to investigators about meetings with Russians during the presidential transition. He cooperated with Mueller under a plea agreement and awaits sentencing.

Michael Cohen, Trump’s former personal lawyer, who pleaded guilty in November 2018 to lying to Congress about a proposed real-estate deal in Moscow. He cooperated with Mueller. Cohen also pleaded guilty in August 2018 in a case prosecuted by the U.S. Attorney's Office in New York to charges related to making six-figure payments as hush money during the campaign to two women who claimed to have had sex with Trump. Cohen was sentenced in December 2018 to three years in prison.

Paul Manafort, Trump’s former campaign chairman, who was convicted in August of tax and bank charges. He pleaded guilty in September to conspiracy against the U.S. to commit money laundering, tax fraud and failing to register to represent foreign interests, and to obstruction of justice. Manafort was convicted of not reporting income and not registering with the Justice Department for representing a pro-Russia faction in Ukraine, and then not reporting millions in income. The obstruction charge stemmed from telling other witnesses to change statements to investigators while he was in custody. Manafort was sentenced 7.5 years in prison.

Rick Gates, who worked for Manafort before and during the campaign and also headed Trump’s inaugural committee. He pleaded guilty in February 2018 to conspiracy and lying to FBI agents and prosecutors. He cooperated with Mueller and awaits sentencing.

Konstantin Kilimnik, who worked with Manafort and Gates in Kiev. He was indicted in June 2018 on charges of conspiracy to obstruct justice.

George Papadopoulos, a foreign policy adviser to the Trump campaign. He pleaded guilty in October 2017 to lying to FBI agents about an overseas professor who claimed to have “dirt” from thousands of emails from Democrat Hillary Clinton and with a Russian woman with ties to Russian officials. He served 14 days in prison and was ordered to pay a $9,500 fine and complete 200 hours of community service.

Alex van der Zwaan, an English lawyer, who pleaded guilty in February 2018 to lying to FBI agents about his work for Manafort and Gates. He served 30 days in prison and was fined $20,000.

Richard Pinedo, of Santa Paula, California, who pleaded guilty in February 2018 to identity fraud for trading in bank account numbers from stolen identities that others used to hide digital payments over the internet. Pinedo was sentenced to six months in prison and ordered to complete 100 hours of community service.

12 Russian nationals who were indicted in July 2018 with conspiracies for hacking Democratic computers to influence the 2016 election. Charges included aggravated identity theft and money laundering.

13 Russian nationals and three entities including the Internet Research Agency that were indicted in February 2018 with conspiracy to defraud the U.S. for interfering with the election. Three were charged with conspiracy to commit wire and bank fraud. Five were charged with aggravated identity theft.

This article originally appeared on USA TODAY: Special counsel Robert Mueller delivers report marking end of investigation into Trump's campaign, Russia