The first defendants in a criminal investigation of Russia's meddling in the 2016 presidential campaign could be taken into custody as soon as Monday, people familiar with the matter said, though the nature and target of the charges couldn't be determined over the weekend.
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Friday, prosecutors led by Robert Mueller, the special counsel in the Russia matter, obtained a grand jury indictment against at least one person. Mr. Mueller's team obtained the charges under seal, the people familiar said.
A spokesman for Mr. Mueller declined to comment. The people familiar with the case declined to identify the person or people who have been charged or to specify what is in the indictment.
The charges would be the first from an investigation that Mr. Mueller took over in May, with his appointment by the Justice Department as a special counsel. That followed President Donald Trump's firing of James Comey as director of the Federal Bureau of Investigation. Mr. Comey had earlier said the agency was investigating whether members of Mr. Trump's 2016 campaign collaborated with Russia to influence the election, and his firing spurred immediate calls to appoint a new person to oversee that investigation.
Mr. Mueller's team, which includes 16 attorneys versed in public-corruption, fraud and national-security matters and more than two dozen FBI agents, has been presenting evidence before a federal grand jury convened in Washington since July.
Besides investigating Russia influence in the election, Mr. Mueller has been investigating whether Mr. Trump obstructed justice in his firing of Mr. Comey, and the business dealings of several former Trump aides, according to people familiar with the matter.
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Mr. Trump and his lawyers have denied any obstruction and any collusion between his campaign and Russia. Mr. Trump has repeatedly labeled the Mueller investigation a "witch hunt." Russia has denied interference in the campaign.
Mr. Trump on Sunday sent several tweets that criticized Democrats and implied the focus on him and Russia was designed to distract from the GOP push to enact a new tax law, which is expected to be unveiled this week.
"All of this 'Russia' talk right when the Republicans are making their big push for historic Tax Cuts & Reform," he tweeted. "Is this coincidental? Not!"
Mr. Mueller also has been probing former Trump campaign manager Paul Manafort, who is the focus of an investigation examining whether he violated tax laws or engaged in illicit financial transactions. In July, FBI agents working with Mr. Mueller raided a home of Mr. Manafort's to obtain documents and other material tied to foreign bank accounts and tax matters.
Mr. Manafort's spokesman, Jason Maloni, has said in the past that Mr. Manafort didn't collude with the Russian government to help Moscow interfere in the 2016 election. He declined to comment Sunday.
Mr. Manafort and Mike Flynn, Mr. Trump's former national security adviser, are also being investigated for potential violations of a law governing the public disclosure of lobbying by foreign powers, according to people familiar with the investigations.
A lawyer for Mr. Flynn declined to comment.
Legal experts said they expect any indictment to be the first in a chain of investigative steps geared toward winning cooperation of potential witnesses in the hopes of building cases against other potential suspects.
"Typically, an investigation such as this is designed to build upon itself," said Steven Levin, a former federal prosecutor. "In other words, the first indictment is intended to lead to both a guilty verdict and another indictment. Usually, this happens through cooperation. So, prosecutors hope to persuade the first defendant to flip on the next defendant. And so on."
The potential charges come at a key moment, with Republicans set to unveil their tax plan, which they hope to pass by Thanksgiving. Some Republicans over the weekend were taking a wait-and-see stance on Mr. Mueller's investigation.
"I have not yet seen any definitive evidence of collusion," Sen. Susan Collins (R., Maine) said in a weekend CBS interview. "I have seen lots of evidence that the Russians were very active in trying to influence the election."
Maine Sen. Angus King, an independent who caucuses with Democrats, and a member of the Senate Intelligence Committee, took issue with one of Mr. Trump's recent tweets that said it was "commonly agreed...that there was NO collusion between Russia and Trump."
"It's certainly not commonly agreed in our committee, and we're the ones doing the investigation," Mr. King said Sunday on CNN. "It's entirely possible we will end up with the investigation at the end finding that there wasn't. But the contrary could also be true."
Some Democrats and legal observers say they are concerned that, as the Mueller investigation starts to pursue criminal charges, the president may seek to pardon anyone charged by Mr. Mueller or even fire the special counsel.
Mr. Trump can't pardon people "if it's an effort to obstruct justice, if it's an effort to prevent Bob Mueller and others from learning about the President's own conduct," Rep. Adam Schiff of California, the top Democrat on the House Intelligence Committee, said Sunday on ABC.
Some allies of Mr. Trump's have suggested Mr. Mueller, a former FBI director under presidents of both parties, isn't impartial.
"If this man's team executes warrants this weekend he should stripped of his authority by @realDonaldTrump. Then HE should be investigated," Sebastian Gorka, a former White House adviser, said in a Friday tweet.
Mr. Trump also has tried to shift the spotlight on his 2016 election rival, Hillary Clinton, saying that her campaign's funding of opposition research that produced a dossier of unverified allegations of Trump-Russia links was a sign that she was working with Russia.
"There is so much GUILT by Democrats/Clinton, and now the facts are pouring out. DO SOMETHING!" he tweeted Sunday.
A Clinton campaign spokesman said last week that he didn't learn about the dossier until after the election but said he would have passed it along to reporters to investigate if he had.
--Kristina Peterson contributed to this article.
Write to Aruna Viswanatha at Aruna.Viswanatha@wsj.com and Del Quentin Wilber at email@example.com