WASHINGTON — The Senate Judiciary Committee has uncovered evidence that Jared Kushner, President Trump’s son-in-law and senior adviser, was forwarded a document about a “Russian backdoor overture” that Mr. Kushner failed to hand over to the panel’s investigators, according to a letter that the committee released on Thursday.

The Senate letter did not say what type of back channel communication the Russians were trying to set up. But it noted that “other parties have produced documents concerning the matter.”

Mr. Kushner also failed to provide investigators with a September 2016 email he was sent about WikiLeaks, nor did he hand over other communications with a Russian-born businessman that were forwarded him, according to the letter. The businessman, Sergei Millian, a former head of the Russian-American Chamber of Commerce, has long claimed to have ties to Mr. Trump and his associates — ties that Mr. Trump’s advisers have said are overstated.

WikiLeaks has been identified by American intelligence agencies as acting as a conduit for information that Russian intelligence operatives had stolen from Democrats during the 2016 presidential campaign. Earlier this week, it was revealed that Donald Trump Jr., the president’s oldest son, had repeated communications with the anti-secrecy group on Twitter.