Senior White House adviser and son-in-law to the president Jared Kushner failed to hand over to Senate investigators emails concerning contacts with WikiLeaks and a "Russian backdoor overture," according to a letter sent by two senior lawmakers.

The letter, released Thursday by Sen. Chuck Grassley, the chairman of the Senate Judiciary Committee, and its ranking Democrat, Sen. Dianne Feinstein, says Kushner failed to turn over "September 2016 email communications to Mr. Kushner concerning WikiLeaks" and other emails pertaining to a "Russian backdoor overture and dinner invite."

The lawmakers said they were seeking the documents that were "known to exist" from other witnesses in the investigation.

"We appreciate your voluntary cooperation with the committee's investigation, but the production appears to have been incomplete," the letter, sent to Kushner's attorney, Abbe Lowell, said. "It appears your search may have overlooked several documents."

Those overlooked documents also included unspecified phone records, according to The New York Times.

In a statement on Thursday, Lowell said he and his client had provided the 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."

"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," Lowell said.

The WikiLeaks emails from September 2016 would coincide with Twitter messages between the organization and Donald Trump Jr., the president's eldest son. U.S. officials believe that WikiLeaks acted as a conduit for emails from the Democratic National Committee and senior Democratic officials who were hacked by Russia.

The Trump administration has repeatedly denied collusion with Russia to influence the outcome of the 2016 election. And less than a month before the election, then-vice presidential candidate Mike Pence, during an interview on Fox News, specifically denied any contacts between WikiLeaks and the campaign.

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