In a heated letter to James Comey released Sunday, Senate Democratic Leader Harry Reid charged that the FBI director is withholding “explosive” information about GOP nominee Donald Trump‘s ties to Russia.

Reid, 76, specifically claimed that he learned — in a conversation with Comey and “other top officials in the national security community” — of the “coordination between Donald Trump, his top advisors and the Russian government.”

Reid’s letter was sent to chide Republican Comey’s recent public announcement that the FBI is taking additional “investigative steps” into Hillary Clinton‘s use of a private email server during her time as secretary of state — news revealed just 11 days before Election Day. Reid said that Comey’s investigation into Clinton, but not Trump, shows “a disturbing double standard.”

But what is the “explosive” evidence against Trump — if any?

The FBI could not immediately be reached for comment on whether such an investigation exists, and Reid’s office did not immediately return PEOPLE’s request for additional comment — although spokesman Adam Jentleson pointed to classified briefings as evidence.

Jentleson told The Washington Post, “There have been classified briefings on this topic. That is all I can say.”

Trump has been linked to the country before, especially following the recent email hacks of the Democratic National Committee and Clinton campaign Chairman John Podesta. The U.S. government has officially accused Russia of orchestrating the cyber attacks in an effort to influence the presidential election.

An October joint statement from the Office of the Director of National Intelligence and the Department of Homeland Security said that “the The U.S. Intelligence Community is confident that the Russian Government directed the recent compromises of e-mails from U.S. persons and institutions, including from U.S. political organizations,” reported The Washington Post. The statement continued, “We believe, based on the scope and sensitivity of these efforts, that only Russia’s senior-most officials could have authorized these activities.”

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Earlier in the year — and before the U.S.’s formal accusation of Russia’s involvement in the hacks — Trump even challenged Russia to further hack Clinton during a July press conference, saying, “I hope you’re able to find the 30,000 emails that are missing.”

“I think you will probably be rewarded mightily by our press.”

At the time, Trump also contradicted his earlier claim that he’d gotten to know Russian President Vladimir Putin “very well” when they were both interviewed for 60 minutes (albeit in different countries). Changing his tune, Trump asserted, “I don’t know who Putin is.”

He also tweeted in July, “For the record, I have ZERO investments in Russia.”

Trump’s former campaign chairman, Paul Manafort, did, however. Manafort was involved in several, multi-million dollar business plans — all unexecuted — with Russian and Ukrainian groups, reported NBC News. He also worked as an adviser and consultant for the Ukrainian presidential campaign of Viktor Yanukovych — who was opposed by the U.S. government because of his ties to Putin. Manafort resigned as Trump’s chairman in August, after the revelation of his ties.

Clinton campaign chairman John Podesta accused another Trump advisor — Roger Stone — of conspiring with WikiLeaks to hurt Clinton’s campaign, according to Fox News.

“I think it’s a reasonable assumption, or at least a reasonable conclusion, that Mr. Stone and the Trump campaign had advance warning about what Assange was going to do,” Podesta charged, citing Stone’s alleged earlier tweet that the 67-year-old’s “time in the barrel” was coming following the Democratic National Committees’s hack.

Reid’s new accusations against Comey are even more controversial in light of previous allegations from the soon-to-be retired senator. During the 2012 presidential election, Reid alleged that Republican candidate Mitt Romney hadn’t paid taxes in 10 years, said the Post. Reid was eventually proved wrong, and he never apologized for the claim, even when charged with attempting to sway the election.

Asked later by the Post if there was a line he “wouldn’t cross” when it came to politics, Reid said: “I don’t know what that line would be.”