Donald Trump’s private company was “actively negotiating” a business deal in Moscow with a sanctioned Russian bank during the 2016 election campaign, according to a memo by Democratic lawmakers investigating possible collusion between the campaign and the Kremlin.

The statement by Democrats on the House intelligence committee, who have had access to internal Trump Organization documents and interviewed key witnesses, raises new questions about the Trump Organization’s financial ties to Russia and its possible willingness to deal with a bank that had been placed under US sanctions.

Trump has personally denied that he ever had business dealings with Russia. In a tweet that was published shortly before his 2017 inauguration, he said he had “nothing to do with Russia – no deals, no loans, no nothing”.

But doubts about the veracity of that statement began to emerge last August, when the New York Times published emails from a longtime business associate of Trump called Felix Sater, who boasted that he had lined up financing for a Trump Tower in Moscow with VTB Bank, which is under US sanctions.

It is not clear from the Democrats’ memo whether the deal they are referring to is the same deal mentioned in Sater’s emails.

In another email obtained by the newspaper, Sater wrote that he would get “Putin on this programme and we will get Donald elected”.

Trump eventually signed a non-binding “letter of intent” for the project to go forward and his attorney, Michael Cohen, told the New York Times that they had discussed the project three times. The Moscow project did not go ahead.

Now the memo released by Democrats suggests investigators have confirmed that the Trump Organization was deeply involved in Moscow talks, and that the talks involved a bank that was under US sanctions. The Democrats did not indicate the source of their information.

The Democrats’ statement came days after Republicans on the House intelligence committee said they had concluded that Trump’s 2016 campaign had not colluded with Russian operatives and that the committee was nearing the end of its investigation.

Adam Schiff, the top Democrat on the committee, called the decision a “capitulation” to the White House. The Democrats then released a 21-page memo detailing some of the committee’s findings, outstanding questions and a long list of witnesses that had yet to be questioned by the committee.

The memo stated that congressional investigators had – among many open questions – not yet determined whether Trump campaign officials had received advanced knowledge of or access to emails that had been hacked by Russians from the Democratic National Committee and Hillary Clinton’s campaign.

The memo stated that there were “critical unanswered questions” about the sources of Trump’s personal and corporate financing, including his relationship with his biggest lender, Deutsche Bank.

It said it needed answers about whether the Russian government, through “business figures close to the Kremlin”, was seeking to launder funds through the Trump Organization, and whether Trump’s financial exposure – through his debt with Deutsche Bank – gave a point of leverage to Russia.

New questions about the Trump Organization’s possible dealings with a Russian bank came on Thursday as it was reported that Robert Mueller, the special counsel who is leading the criminal investigation into the Trump campaign and possible collusion, has issued a subpoena demanding that the Trump Organization turn over documents.

The subpoena, which was reported by the New York Times, included a demand for documents related to Russia, according to two sources cited by the newspaper. The Trump Organization did not immediately respond to a request for comment.

The news was significant because it showed that Mueller is definitively including the president’s private business in his investigation and examining possible financial ties to Russia.