By Warren Strobel, Dustin Volz and Jonathan Landay

WASHINGTON (Reuters) - A Russian propaganda arm oversaw a criminal and espionage conspiracy to tamper in the 2016 U.S. presidential campaign to support Donald Trump and disparage Hillary Clinton, said an indictment released on Friday that revealed more details than previously known about Moscow's purported effort to interfere.

The office of U.S. Special Counsel Robert Mueller charged 13 Russians and three Russian companies, including St. Petersburg-based Internet Research Agency known for its trolling on social media. The official who oversees Mueller's work said the investigation was not finished.

(Indictment: http://tmsnrt.rs/2EvtgMh)

The court document said those accused "had a strategic goal to sow discord in the U.S. political system, including the 2016 U.S. presidential election."

The indictment said Russians adopted false online personas to push divisive messages; traveled to the United States to collect intelligence, visiting 10 states; and staged political rallies while posing as Americans. In one case, it said, the Russians paid an unidentified person to build a cage aboard a flatbed truck and another to wear a costume "portraying Clinton in a prison uniform."

The surprise 37-page indictment could alter the divisive U.S. domestic debate over Russia's meddling, undercutting some Republicans who, along with Trump, have attacked Mueller's investigation.

"These Russians engaged in a sinister and systematic attack on our political system. It was a conspiracy to subvert the process, and take aim at democracy itself," said Paul Ryan, Republican Speaker of the House of Representatives.

The indictment is silent on the question of whether the Trump campaign colluded with the Kremlin, which Mueller is investigating.

In a Tweet on Friday, Trump gave his most direct acknowledgement that Russia had meddled in the election, which he has frequently disputed.

"Russia started their anti-US campaign in 2014, long before I announced that I would run for President. The results of the election were not impacted. The Trump campaign did nothing wrong - no collusion!" Trump wrote.

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Russian Foreign Ministry spokeswoman Maria Zakharova called the allegations that Russian nationals interfered in U.S. elections "absurd" and ridiculed the idea that so few could undermine U.S. democracy.

"13 against the billions' budgets of the secret services?" she asked in a Facebook post.

The accused Russians are unlikely to be arrested or to appear in a U.S. court on the charges, which include conspiracy to defraud the United States, wire fraud, bank fraud and identity theft.

ECHOES OF INTELLIGENCE REPORT

The indictment broadly echoes the conclusions of a January 2017 U.S. intelligence assessment, which found that Russia had meddled in the election, and that its goals eventually included aiding Trump. In November 2016, Trump won a surprise victory over Democratic Party candidate Clinton.

Mueller's indictment did not tie the meddling effort to the Russian government. But the earlier U.S. intelligence assessment said Russian President Vladimir Putin ordered a campaign to influence the U.S. election.

Trump has never unequivocally accepted the U.S. intelligence report and has denounced Mueller's probe into whether his campaign colluded with the Kremlin as a "witch hunt."

Some of those charged, posing as Americans, "communicated with unwitting individuals associated with the Trump campaign," the indictment said.

Last year, Mueller charged Trump's former campaign manager and his deputy with money-laundering and other crimes, and accepted guilty pleas from two former foreign policy aides for lying to the FBI.

Friday's indictment of the Russians, coupled with the FBI disclosure that it failed to heed a warning about the Florida high school shooter, were blows to the White House, still reeling from the fallout of a scandal involving a former aide accused of domestic abuse by two ex-wives.

U.S. stocks had been up over half a percent but lost nearly all of that after the indictment came out.

'CONSPIRATORS'

U.S. Deputy Attorney General Rod Rosenstein told a press conference: "The defendants allegedly conducted what they called information warfare against the United States, with the stated goal of spreading distrust towards the candidates and the political system in general."

The indictment describes a sophisticated, multi-year and well-funded operation, dubbed "Project Lakhta," by Russian entities to influence the election, beginning as early as May 2014.

Russians unlawfully used stolen social security numbers and birth dates of Americans to open accounts on the PayPal digital payment platform and to post on social media using those fake identities, the indictment said.

Mueller also on Friday reached an agreement with an American named Richard Pinedo, who pled guilty to aiding and abetting interstate and foreign identity fraud by creating, buying and stealing hundreds of bank account numbers that he sold to individuals to use with large digital payment companies.

According to a source familiar with the indictments, Pinedo is the person cited in the document as helping the Russian conspirators launder money, as well as purchase Facebook ads and pay for rally supplies, through PayPal Holdings Inc..

Pinedo's attorney, Jeremy I. Lessem, said in a statement that "Mr. Pinedo had absolutely no knowledge of the identities and motivations of any of the purchasers of the information he provided."

The Russians sought to measure the impact of their online social media operations, tracking the size of U.S. audiences reached through posts and other types of engagement, such as likes, comments and reposts, according to the indictment.

Facebook said in a statement that it had previously disclosed the Internet Research Agency's activity on its platform. "We know we have more to do to prevent against future attacks," said Joel Kaplan, Facebook's vice president of global policy.

Twitter, whose platform also was used, declined to comment on the indictment.

The Internet Research Agency was registered with the Russian government as a corporate entity in July 2013 and St. Petersburg "became one of the organization’s operational hubs," for the project, the indictment said.

The organization employed hundreds of people, ranging from creators of fictitious person to technical experts, and by September 2016, its monthly budget was in excess of $1.2 million, the court document said.

Director of National Intelligence Dan Coats told the Senate Intelligence Committee on Tuesday that he had already seen evidence Russia was targeting U.S. elections in November, when Republican control of the House of Representatives and Senate are at stake, plus a host of positions in state governments.

The indictment said the Russians it charged tried to destroy evidence of their crimes.

For example, in September 2017, one of the defendants wrote an email to a family member stating: "We had a slight crisis at work: the FBI busted our activity (not a joke). So, I got preoccupied with covering tracks together with colleagues."

The email continued: "I created all these pictures and posts, and the Americans believed that it was written by their people."

(Reporting by Warren Strobel, Dustin Volz, Jonathan Landay; additional reporting by Lisa Lambert, Patricia Zengerle, Mark Hosenball, David Shepardson, David Alexander, Steve Holland, David Ingram, Noel Randewich and; Jack Stubbs and Christian Lowe in Moscow; editing by Mary Milliken and Grant McCool)