Attorney General William Barr on Sunday released the “principal conclusions” of Special Counsel Robert Mueller’s long-awaited investigation into Russian meddling in the 2016 presidential race and possible collusion with Trump associates. The four-page letter to Capitol Hill lawmakers stated that Mueller did not establish evidence that President Trump’s team or any associates of the Trump campaign had conspired with Russia and is not recommending additional indictments.

The report's release came on the heels of two separate sentencings this month for Trump's former campaign chairman Paul Manafort, who will serve nearly seven years in prison. On March 13, a federal judge sentenced Manafort, 69, to three and a half years in prison. Less than a week earlier, on March 7, Manafort was sentenced to 47 months in prison after being convicted of eight bank and tax fraud charges last August.

Dozens of people have either been indicted or convicted, or they have entered a guilty plea as part of the investigation, which began in May 2017. Several former Trump campaign associates – Michael Flynn, Paul Manafort, Richard Gates, Roger Stone and George Papadopoulos – are among the scalps via Mueller's team, while at least 25 are Russian officials.

Here's a closer look at those who have faced charges throughout Mueller's probe.

Roger Stone

Roger Stone, a longtime political adviser to Trump, was indicted on charges of obstruction, making false statements and witness tampering by Mueller's office.

Stone, 66, was arrested in Florida on Jan. 25, a spokesperson for Mueller's office confirmed. For months, Stone had warned he could be indicted, publicly saying he believed Mueller was investigating whether he had knowledge of WikiLeaks releasing hacked emails of Democrats during the 2016 campaign. Stone has repeatedly denied doing so.

The indictment alleges Stone worked to obstruct the Senate Intelligence Committee’s investigation into Russian interference in the 2016 election by making false statements to the committee, denying he had records sought by the committee and persuading a witness to provide false testimony.

Michael Flynn

Michael Flynn, who served as Trump’s national security adviser for less than a month before resigning, pleaded guilty in December 2017 to making false statements to the FBI.

Flynn reportedly lied about his talks with Russia's ambassador to Washington. In late 2016, while former President Barack Obama was still in office, the two allegedly spoke about the U.S. sanctions imposed on Russia.

This raised concerns that Flynn, a retired Army lieutenant general, misled Trump officials about his conversations with Russian officials.

Paul Manafort

The special counsel filed a 32-count indictment on Feb. 22, 2018, against former Trump campaign chairman Paul Manafort and aide Rick Gates, accusing the pair of tax evasion and bank fraud.

The indictment accused Manafort and Gates of doctoring documents to inflate the income of their businesses and then using those fraudulent documents to obtain loans. It also accused Manafort of evading taxes from 2010 through 2014 and, in some of the years, concealing his foreign bank accounts.

Manafort turned himself into federal authorities in the fall of 2018. The 69-year-old served as Trump’s campaign manager for a few months in 2016. Gates, Manafort's business associate, also turned himself in at the time.

On June 15, 2018, Manafort was jailed after a federal judge revoked his $10 million bail based on new witness tampering charges brought by Mueller.

Manafort was found guilty of eight financial crimes on Aug. 21, 2018, in the first trial victory of the special counsel investigation into the president's associates. He later pleaded guilty in a second case and agreed to cooperate with Mueller's team.

On March 7, he was sentenced to 47 months in prison. He was credited with the 9 months he had already spent in prison, so he will only have to serve 38 months or just more than 3 years.

Days later, on March 13, a federal judge sentenced Manafort to an additional three and a half years of prison. The sentence is on top of the roughly four-year prison sentence Manafort received last week in a separate criminal case in Virginia.

Additionally, Manafort was indicted in New York on state charges on March 13. The indictment -- which is seen as a strategy for preventing a potential presidential pardon -- was unsealed in Manhattan and accuses Manafort of conducting a yearlong residential mortgage fraud scheme that netted millions of dollars.

Richard Gates

Richard Gates was named alongside Manafort in charges brought by the special counsel. He is accused of 11 counts related to filing false income tax returns and three counts of failure to report foreign bank and financial accounts.

Gates pleaded guilty in February 2018 to federal conspiracy and false-statements charges.

A superseding criminal complaint says Gates was charged with conspiracy against the United States between 2006 and 2017.

George Papadopoulos

A former foreign policy adviser to Trump’s presidential campaign, George Papadopoulos pleaded guilty in 2017 to making false statements to the FBI regarding “the timing, extent and nature of his relationships and interactions with certain foreign nationals whom he understood to have close connections with senior Russian government officials,” according to court documents.

He also reportedly tried to set up meetings between Russian and Trump campaign officials on various occasions.

Papadopoulos was sentenced in September 2018 to 14 days in prison, 13 months of supervised release, 200 hours of community service and a $9,500 fine for lying to the FBI during the Russia probe.

“My entire life has been turned upside down, I hope to have a second chance to redeem myself,” he said during his sentencing.

Michael Cohen

Formerly Trump's attorney, Michael Cohen was sentenced to three years in prison after pleading guilty to multiple charges that arose from two separate investigations – one by federal prosecutors in New York, and the other by Mueller.

Cohen pleaded guilty to tax evasion, campaign finance violations and lying to Congress about a Trump real estate project in Moscow. He was sentenced on Dec. 12, 2018.

Alex van der Zwaan

Mueller's team charged Dutch attorney Alex van der Zwaan for lying to federal investigators in the Russia probe in federal court in February 2018. He pleaded guilty and was sentenced to 30 days in prison and given a $20,000 fine in April.

Van der Zwaan was released from prison in June and turned over to Immigration and Customs Enforcement officials.

According to charging documents, a law firm hired by the Ukraine Ministry of Justice in 2012 employed van der Zwaan. He admitted to lying about his interactions with Gates.

The charge against van der Zwaan did not involve election meddling or the Trump campaign's operations. It stemmed from the special counsel's investigation into a covert Washington lobbying campaign Manafort and Gates are accused of directing on behalf of pro-Russian Ukrainian interests.

Richard Pinedo

Richard Pinedo, a California man who sold bank accounts to Russians meddling in the election, pleaded guilty in February 2018 to using stolen identities to set up the accounts. He was sentenced in October to six months in prison, six months in home confinement and two years of supervised release.

The U.S. government said Pinedo did not know that he was dealing with Russians when he sold the accounts. Since his arrest, Pinedo has provided investigators with "significant assistance" in identity theft probes, prosecutors said.

During his sentencing, Pinedo told the judge he took "full responsibility" and understood "there needs to be consequences" for his actions. Federal sentencing guidelines called for Pinedo to serve between 12 and 18 months behind bars, but prosecutors did not recommend a particular sentence, noting his cooperation with officials.

13 Russian nationals

A grand jury indicted 13 Russian nationals and three Russian companies in February 2018 for allegedly interfering in the 2016 election. In the case, Mueller detailed a sophisticated plot to wage “information warfare” on the U.S.

The indictment was the first to be brought against Russian nationals in Mueller's investigation.

However, the Justice Department said the indictment does not allege that the interference changed the outcome of the election.

"There is no allegation in this indictment that any American was a knowing participant in this illegal activity," said Deputy Attorney General Rod Rosenstein, who oversaw the special counsel probe.

12 Russian intelligence officers

The Justice Department in July 2018 announced that 12 Russian intelligence officers were indicted for allegedly hacking the Democratic National Committee, the Democratic Congressional Campaign Committee and Hillary Clinton's campaign during the 2016 election.

All 12 are members of GRU, the Russian intelligence agency.

Fox News' Ann Schmidt, Adam Shaw, Samuel Chamberlain, Jake Gibson, Alex Pappas, Brooke Singman and The Associated Press contributed to this report.