Russian agents sought secret U.S. Treasury Department records on Clinton backers during the 2016 campaign, just weeks before the Trump Tower meeting where a Russian attorney offered “dirt” on Hillary Clinton that allegedly involved those same supporters.

Treasury Department officials used a Gmail back channel with the Russian government, according to whistleblowers, as the Kremlin sought sensitive financial information about the Ziff brothers, reported Buzzfeed News.

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The billionaire investors Dirk, Edward, and Daniel Ziff had run afoul of the Kremlin, and Russia’s financial crimes agency — which is overseen by a former KGB officer and schoolmate of president Vladimir Putin — asked U.S. officials for documents related to executives from the Anti-Defamation League and the National Council of Jewish Women, as well as other Kremlin opponents living in London and Kiev.

The request on the Ziff brothers came just weeks before attorney Natalia Veselnitskaya visited Trump Tower on June 9, 2016, for a meeting with Donald Trump Jr., Jared Kushner and Paul Manafort, where she reportedly tried to link the financiers to an alleged scandal involving the Clinton Foundation.

Documents obtained by Buzzfeed show Treasury officials used their government email accounts to exchange messages with private Hotmail and Gmail accounts set up by the Russians, rather than communicating through a secure network.

Analysts in the department warned supervisors in 2016 that Russians were conducting “fishing expeditions,” and they feared Treasury’s internal systems could be compromised by viruses contained from the unofficial Russian accounts.

However, staffers continued using the Gmail back channel until 2017, despite repeated warnings that Russia could be trying to obtain sensitive financial records to spy on or recruit targets in the U.S.

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The Treasury Department declined to say why its officials were communicating with the unofficial private accounts while Russia was sending suspicious requests.

“Treasury does not discuss or comment on confidential communications with foreign governments, including to confirm whether or not they have occurred,” a spokesperson said. “We have notified our Office of the Inspector General of these allegations.”

The Financial Crimes Enforcement Network reported the back channel to the department’s counterterrorism unit in July and August 2016 and requested an investigation, but sources told Buzzfeed that little was done.