WASHINGTON—President Donald Trump’s lawyers on Friday released a two-month-old letter stating that 10 years of his tax returns show little income, investments or debt from Russian sources beyond items already known to the public.

Unlike previous presidents or candidates going back decades, Mr. Trump hasn’t released any of his tax returns, and the one-page letter makes no suggestion that more releases are coming. The letter provides no independent verification of the president’s business relationships, and its conclusions are carefully worded and incomplete, tax lawyers said.

Democrats have been pressing Mr. Trump to disclose any financial ties to Russia amid investigations of possible links between his presidential campaign and the country, particularly in light of this week’s firing of Federal Bureau of Investigation Director James Comey.

The one-page letter dated March 8 states there are a “few exceptions” to the statement that Mr. Trump had no Russian sources of income. Those include as much as $12.2 million in income from the 2013 Miss Universe pageant, which took place in Russia, and the 2008 sale, for $95 million, of a Florida estate to a Russian billionaire. Mr. Trump had mentioned those two links to Russia during an NBC interview broadcast Thursday, when the president was asked by news anchor Lester Holt whether he or his family members had accepted loans or investments from any Russians.

The letter also states “it is likely” over the years that Mr. Trump and his businesses sold goods, services and assets—including condominiums—to Russians. That income, the lawyers wrote, wouldn’t have been identified as Russian in Mr. Trump’s financial statements, and the amounts, they wrote, are “immaterial.”