The arrest Friday of longtime Trump adviser Roger Stone, on charges brought by special counsel Robert Mueller, marks the 34th person and sixth associate of President Trump to be accused of wrongdoing during the 20-month investigation into Russian meddling in the 2016 presidential election.

Mr. Mueller’s roughly $25 million investigation has publicly lodged more than 100 criminal counts against 38 people and businesses, including 25 Russian nationals and three companies.

Charges have ranged from crimes of direct election interference to false statement and financial fraud allegations unrelated to the presidential campaign.

The investigation hasn’t offered evidence that Trump campaign advisers colluded with Russian officials to tilt the election, but it has racked up five guilty pleas from five former Trump aides.

Trump campaign associates: Roger Stone

Mr. Stone, who has worked as a political consultant for Mr. Trump since the 1980s, was arrested and charged with one count of obstruction of justice, five counts of false statements and one count of witness tampering.

Prosecutors say Mr. Stone obstructed investigations by the House Permanent Select Committee on Intelligence and the FBI into Russian election interference by making “multiple false statements” about his interactions with an organization widely believed to be WikiLeaks.

The emails hacked from Hillary Clinton’s presidential campaign and the Democratic National Committee before the election were stolen by Russian agents and released by WikiLeaks.

Mr. Stone vowed to fight the charges, saying he intends to plead not guilty and go to trial.

Michael Cohen

Mr. Trump’s longtime fixer Michael Cohen was sentenced last month to three years in prison, the harshest sentence so far in the Mueller investigation. He is scheduled to report to a federal prison on March 6.

Cohen pleaded guilty to campaign finance violations for arranging hush payments to two women who said they had affairs with Mr. Trump. Prosecutors said the payments were made in an effort to influence the election and that Cohen “acted in coordination with and at the direction of Individual-1,” widely believed to be Mr. Trump.

He also pleaded guilty to making false statements to Congress about a defunct plan to build a Trump Tower in Moscow. He initially claimed that the plan ended early in 2016, when in fact it was an active project into the summer of that year.

Cohen is cooperating with Mr. Mueller. He has spent more than 70 hours in meetings with the special counsel’s team.

The Senate Select Committee on Intelligence subpoenaed Cohen on Thursday to testify in mid-February. Cohen postponed his scheduled Feb. 7 appearance before the House Oversight Committee, citing “threats” by Mr. Trump and his attorney Rudolph W. Giuliani.

Paul Manafort

Former Trump campaign chairman Paul Manafort was found guilty of eight financial fraud crimes by a federal jury in Alexandria, Virginia. He pleaded guilty to one count of conspiracy to obstruct justice and one count of conspiracy against the U.S. in a separate case in the District of Columbia.

All of the charges stemmed from crimes Manafort committed before he joined Trump’s campaign and are unrelated to election interference.

Manafort’s plea deal included an agreement to cooperate with Mr. Mueller’s team. In November, Mr. Mueller sought to nix that deal, saying Manafort lied to investigators on “a variety of subject matters.” Attorneys for Manafort insist he did not lie intentionally, but rather struggled to remember facts of events from a long time ago or during the pressure of the presidential campaign. A federal judge in the District on Friday ordered a sealed Feb. 4 hearing on the matter.

Michael Flynn

Michael Flynn, Mr. Trump’s onetime national security adviser, pleaded guilty in December 2017 to lying to the FBI about his conversations with the former Russian ambassador to the U.S. He is perhaps the most high-profile person indicted by Mr. Mueller.

Mr. Trump fired him in February 2017, saying Flynn had lied to administration officials, including Vice President Mike Pence.

Mr. Mueller said in court filings that Flynn had provided “substantial cooperation” and recommended no jail time during his sentencing hearing in December. However, the sentencing was delayed because the federal judge wanted more information about the scope of Flynn’s cooperation.

The judge reprimanded Flynn during the hearing, saying he couldn’t hide his “disgust and disdain” over the former administration official’s actions.

George Papadopoulos

A former Trump campaign adviser, George Papadopoulos pleaded guilty in October 2017 to making false statements to the FBI about his contacts with Russia.

The indictment said Papadopoulos made at least six attempts to set up a meeting between the Trump campaign and a Russian agent who he believed had dirt on Mrs. Clinton, using as conduits a London-based professor and a Russian woman who claimed to be Russian President Vladimir Putin’s niece.

Papadopoulos served a 14-day jail sentence.

Rick Gates

Rick Gates was indicted with Manafort on charges of conspiracy against the U.S., making false statements and bank fraud. A longtime business associate of Manafort, he worked on the Trump campaign even after his former boss departed.

Shortly after his indictment, Gates pleaded guilty to one charge of lying to investigators and one charge of conspiracy. He agreed to cooperate with Mr. Mueller and was the prosecution’s star witness against Manafort in the Virginia financial fraud case. While on the stand, Gates confessed to embezzling from Manafort to fund an extramarital affair.

A sentencing date has not been scheduled.

13 Russian nationals, three companies

In February 2018, 13 Russian citizens and three companies were indicted on conspiracy and identity theft charges.

The Justice Department said they were part of “troll farms” that used internet bots to spread fake news and political propaganda on social media sites such as Facebook and Twitter.

Given the lack of an extradition agreement with Russia, the individuals have largely ignored the proceedings. One of the companies, Concord Management, has pleaded not guilty and mounted a legal challenge to the indictment. The case is pending in the District of Columbia.

12 Russian intelligence officers

The Justice Department in July announced charges against 12 Russian intelligence officers for hacking the Democratic National Committee emails prior to the 2016 election.

All 12 are members of the GRU, the Russian equivalent to the CIA. They are accused of conspiring to interfere in the election by hacking computers and releasing stolen documents.

The charges mark the first time the special counsel directly accused Russians of meddling in the election. They will likely never see a U.S. courtroom.

The others: Richard Pinedo

Richard Pinedo pleaded guilty to one count of identity fraud in February 2018, one day after Mr. Mueller charged the 13 Russians and three companies.

The California businessman sold phony bank accounts designed to circumvent security measures from online payment companies such as PayPal. Russians used those accounts to buy ads on Facebook and other social media sites.

The indictment says Pinedo was not directly involved in setting up the accounts with fake identities, but rather “willfully and intentionally avoided learning about the use of the stolen identities.”

In October, he was sentenced to six months in prison.

Konstantin Kilimnik

Another Manafort associate, Konstantin Kilimnik, was charged in June with witness tampering ahead of Manafort’s trial in the District.

Mr. Mueller says Mr. Kilimnik worked with Manafort, repeatedly contacting two people with whom they worked in the past “to secure materially false testimony” in the case.

Mr. Kilimnik’s case is pending. He lives in Russia and is unlikely to face a trial in the United States.

Alex van der Zwaan

Alex van der Zwaan pleaded guilty in February to one count of making false statements to the FBI abut his contacts with Gates.

A Dutch lawyer living in the United Kingdom, he worked with Manafort and Gates to produce a report favorable to a Russian-backed politician in Ukraine.

Mr. Van der Zwaan was sentenced to 30 days in jail. With his sentence complete, he was deported to the United Kingdom last year.

Four people have been charged with crimes related to the Mueller investigation but prosecuted by others in the Justice Department. They are two former business associates of Flynn, Bijan Kian and Ekim Alptekin, who are accused of conspiring to violate federal lobbying rules; Maria Butina, an accused Russian spy who pleaded guilty to conspiring to act as a foreign agent; and Sam Patten, a Republican lobbyist who pleaded guilty to violating foreign lobbying laws.