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

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.”

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 Trump and his associates — ties that Trump’s advisers have said are overstated.

WikiLeaks has been identified by U.S. 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.

The Senate panel said that Kushner has also not produced phone records that investigators believe exist, although the letter gave no specifics about the records. The letter was signed by Sens. Charles E. Grassley, R-Iowa, and Dianne Feinstein, D-Calif., the two senior members of the Judiciary Committee.

“We appreciate your voluntary cooperation with the committee’s investigation, but the production appears to have been incomplete,” said the letter, which was sent to Kushner’s lawyer, Abbe Lowell.

“It appears that your search may have overlooked several documents,” it stated.

In a statement, Lowell said today that Kushner had responded to all the committee’s requests.

“We provided the Judiciary Committee with all relevant documents that had to do with Mr. Kushner’s calls, contacts or meetings with Russians during the campaign and transition, which was the request,” Lowell said. “We also informed the committee we will be open to responding to any additional requests and that we will continue to work with White House Counsel for any responsive documents from after the inauguration.”

Over the summer, Grassley and Feinstein said they intended to conduct a bipartisan inquiry into how the Justice Department and FBI handled the investigations into Trump’s connections to Russia, as well as into the firing of James B. Comey, the former FBI director.

After sending initial requests for information to the president’s associates and conducting interviews — including a closed-door interview with Donald Trump Jr. — tension arose between Republican and Democratic investigators about who should be questioned and what documents should be sought.

The Senate letter also pressed Lowell on why he has refused to hand over a copy of a government document that Kushner completed to obtain a security clearance. The document Kushner submitted — known as an SF-86 — has been amended at least twice, after he failed to disclose meetings he had with foreign leaders, including numerous Russians.

Among the meetings that Kushner did not disclose was a June 2016 gathering at Trump Tower that included Donald Trump Jr., Paul J. Manafort, then the campaign chairman, and Russians who had promised “dirt” on Hillary Clinton.

“You declined to produce documents requested from the committee from your SF-86, on the basis that the documents are confidential and have been submitted to the FBI for its review,” the letter stated. “However, if Mr. Kushner or his counsel retained copies of the forms, you should produce them.”