– The Justice Department special counsel has evidence that Donald Trump’s personal lawyer and confidant, Michael Cohen, secretly made a late-summer trip to Prague during the 2016 presidential campaign, according to two sources familiar with the matter.

Confirmation of the trip would lend credence to a retired British spy’s report that Cohen strategized there with a powerful Kremlin figure about Russian meddling in the U.S. election.

It would also be one of the most significant developments thus far in Special Counsel Robert Mueller’s investigation of whether the Trump campaign and the Kremlin worked together to help Trump win the presidency.

Cohen has denied for months that he has ever been in Prague or colluded with Russia during the campaign. Neither he nor his lawyer responded to requests for comment.

It’s unclear whether Mueller’s investigators also have evidence that Cohen met with a prominent Russian — purportedly Konstantin Kosachev, an ally of Russian President Vladimir Putin — in the Czech capital. Kosachev, who chairs the Foreign Affairs Committee of a body of the Russian legislature, the Federation Council, also has denied visiting Prague in 2016. This month, Kosachev was among 24 high-profile Russians whom the U.S. sanctioned in retaliation for Russia’s meddling.

But investigators have traced evidence that Cohen entered the Czech Republic through Germany, apparently in August or early September 2016 as the former spy reported, said the sources. Cohen wouldn’t have needed a passport for such a trip, because both countries are in the Schengen Area, in which 26 nations have open borders.

Peter Carr, a spokesman for Mueller’s office, declined to comment.

Unconfirmed reports of a clandestine Prague meeting came to public attention in January 2017 with the publication of a dossier purporting to detail the Trump campaign’s interactions with Russia — a series of reports that former British MI6 officer Christopher Steele gathered from Kremlin sources for Trump’s political opponents, including Democrat Hillary Clinton’s campaign and the Democratic National Committee. Cohen’s alleged communications with the Russians were mentioned several times in Steele’s reports, which he shared with the FBI.

When the news site Buzzfeed published the entire dossier Jan. 11, 2017, Trump denounced the news organization as “a failing pile of garbage” and said the document was “false and fake.”

Cohen, in a Twitter post, said, “I have never been to Prague in my life.”

In the ensuing months, Cohen allowed Buzzfeed to inspect his passport and, again on Twitter, said, “The Russian dossier is WRONG!”

Democratic investigators for the House and Senate intelligence committees, which are conducting parallel probes into Russia’s election interference, also are skeptical about whether Cohen was truthful about his 2016 travels to Europe when they interviewed him last October, two people familiar with those investigations said this week. The investigators intend to press Cohen for more information, the sources said.