A review of President Donald Trump's tax returns from the past 10 years showed no income from Russian sources outside of a few exceptions, and indicated he did not owe money to Russian lenders, his lawyers said in a letter released by the White House on Friday.

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The letter, dated March 8, said the tax returns did not reflect any Trump income from Russian sources, aside from money earned during the 2013 Miss Universe pageant in Moscow and the 2008 sale of a Florida estate to a Russian billionaire.

Trump's businesses also may have earned "immaterial" amounts of money from Russians over the years through ordinary sales transactions, including condominium rentals or sales, hotel rooms, rounds of golf, or sales of Trump-licensed products, Sheri Dillon and William Nelson, attorneys at Morgan Lewis and Bockius, said in the letter.

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The tax returns also did not reflect any payments by Trump or his businesses to Russian lenders, any Trump investments in Russian firms or any investment by Russians in Trump's companies, the letter said.

Morgan Lewis, a global law firm with 30 offices in cities around the world including New York and Moscow, was named Russia Law Firm of the Year in 2016 by London-based Chambers and Partners, a firm that ranks lawyers and law firms.

The release of the letter by Trump's long-time attorneys came as the president sought to calm the furor caused by his firing this week of FBI Director James Comey, who was leading an investigation into possible collusion between the Trump presidential campaign and Russia to influence the U.S. election.

Comey's firing on Tuesday and the White House's shifting explanations about the rationale for the move have raised concerns among Democrats and others that the White House was trying to blunt the FBI probe.

Trump and the White House have denied any such motive, but Trump told NBC on Thursday that the Russia probe was on his mind when he made the decision to fire the FBI director.

Republican Senator Lindsey Graham said on Tuesday he wanted to know more about Trump's business dealings with Russia and intended to examine whether any connections existed between Russia and the Trump campaign or the president's associates.

The White House welcomed the fact-finding effort by Graham and a subcommittee he chairs.

"He has no business in Russia; he has no connections to Russia," White House spokesman Sean Spicer told reporters, adding that the president had directed his law firm to send Graham a certified letter stating as much.

(Reporting by Steve Holland; Writing by David Alexander)