The NRA was Trump's biggest financial backer, spending more than $US30 million on his behalf.

The committee Democrats pointed to connections between the NRA and Alexander Torshin, the deputy governor of Russia's central bank who also is a close ally of Russian President Vladimir Putin, and Maria Butina, a Torshin protege who founded the Russian group Right to Bear Arms.

Torshin attended several NRA conventions in the United States. Butina and Paul Erickson, a Trump supporter and adviser, formed a limited liability company in February 2016 in South Dakota, where Erickson lives. Erickson told McClatchy the firm was created in case Butina needed financial help with her graduate studies at American University in Washington.

Torshin and Butina "have longstanding ties" to former NRA President David Keene, the report said. Erickson told McClatchy last year that he accompanied Keene on a 2013 trip to Moscow to attend an early conference of the Right to Bear Arms.

"During the campaign, Mr. Torshin, Ms. Butina, and their intermediaries repeatedly offered the campaign back channels to Russia and relayed requests from President Putin to meet with Mr. Trump," the Democratic report said.

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Much of the report was devoted to questions about the June 9, 2016 Trump Tower meeting between Donald Trump Jr and other Trump campaign officials and several Russians that had been advertised as an opportunity for the campaign to collect dirt on Democratic presidential candidate Hillary Clinton.

Feinstein sent letters to Butina, Keene and Erickson requesting documents and asking them to appear for interviews, but they refused to cooperate, the report said.

Thus, "the extent of Russia's use of the NRA as an avenue for connecting with and potentially supporting the Trump campaign needs examination," said the document.

The NRA, in recent letter exchanges with Democratic Senator Ron Wyden of Oregon, said it received only $US2500 in donations from Russians during the 2016 election cycle and that those funds were not used in the election. But it declined to disclose the sources of so-called "dark" contributions whose identities are not required to be made public or whether it knows the trues sources of all those funds.

The NRA was Trump's biggest financial backer, spending more than $US30 million on his behalf.

Asked for comment, NRA spokesman Andrew Arulanandam said of the report: "This is the same tired old soup reheated."

But CNN reported April 27 that the NRA has been culling years of documents related to its communications with Torshin in anticipation of an investigation. The group also has said it is re-examining its relationship with Torshin, who was among 17 Russian government officials recently hit with Treasury Department sanctions that bar Americans from doing business with them.

The current status of the FBI inquiry, part of Justice Department special counsel Robert Mueller's broad counterintelligence and criminal inquiries into Russia's election interference, is unclear.