WASHINGTON (Reuters) - Special Counsel Robert Mueller’s probe of Russian interference in U.S. politics is over, but prosecutors are investigating two prominent Washington lobbyists for their work for the former pro-Russian government of Ukraine, according to three sources close to the probe.

Slideshow ( 3 images )

In a sign that evidence from Mueller’s 22-month investigation may yet ensnare more prominent Washington figures, federal prosecutors in Washington cited a former U.S. congressman “working for the government of Ukraine” in charges filed Thursday against former Obama administration official Greg Craig.

Craig pleaded not guilty on Friday to lobbying violations and making false statements.

The former congressman is not named in Craig’s indictment, but other filings by his former law firm cite identical work done for the firm by Vin Weber, a lobbyist with Mercury Public Affairs, who was a U.S. representative from Minnesota from 1981 to 1993.

Weber disclosed his work for a Ukrainian think tank, but he did not register as a foreign agent for the government of Ukraine under a the 1938 Foreign Agents Registration Act (FARA), which would have required him to spell out his activity in detail, according to a Reuters review of lobbying records.

The Department of Justice is stepping up FARA enforcement after decades of inactivity. Mueller’s investigation has not led to criminal conspiracy charges against Trump or his former associates, but it has exposed previously hidden foreign lobbying activity by some of Washington’s most prominent lobbyists and prompted a flurry of new registrations since 2017. U.S. Attorney General William Barr is expected to release a redacted version of Mueller’s findings next week.

Three sources familiar with the investigation said federal prosecutors in Manhattan are examining work done for Ukraine by both Weber and Tony Podesta, brother of John Podesta, a top official of former Secretary of State Hillary Clinton’s 2016 Democratic presidential campaign and a former White House adviser.

A spokesperson for Weber and for Mercury declined to comment. Tony Podesta also declined to comment.

The FARA crackdown has factored in cases against Craig, Craig’s former law firm Skadden Arps and Paul Manafort, the former Trump campaign chairman now serving a 7-1/2 year prison sentence for lobbying and financial crimes.

Other lobbyists have rushed to comply with the law. As of April, 446 separate entities registered with the Justice Department, up from 387 at the end of 2016.

Tony Podesta disclosed his lobbying work for a Brussels-based think tank, the European Centre for a Modern Ukraine, to Congress. But he did not register with the Justice Department as a foreign agent for the government of Ukraine.

Podesta’s work for the Ukrainian government came to light in 2017 after Mueller’s team charged Manafort with lobbying violations. Those revelations prompted Podesta to step down in October 2017 from his Podesta Associates lobbying firm, which collapsed shortly after.

Sources familiar with the investigation said that investigations into Ukraine-related lobbying by Craig, Podesta and Weber were all initially being pursued by the Manhattan-based U.S. Attorney’s office for the Southern District of New York, but the probe related to Craig was transferred to federal prosecutors in Washington.

Craig denies misleading the Justice Department about his activity in Ukraine, saying in a statement on Thursday that the effort to prosecute him was “unprecedented and unjustified.”

According to the indictment, Craig was hired to write a report for the Ukrainian Justice Ministry about the prosecution of former Ukraine Prime Minister Yulia Tymoshenko.

Prosecutors say Craig concealed the fact that the project was largely funded by Viktor Pinchuk, a Ukrainian oligarch who was close to Bill and Hillary Clinton but also donated to the Trump Foundation after Donald Trump made a September 2015 speech in Kiev.