Sen. Charles Grassley (R-Iowa) and Sen. Dianne Feinstein (D-Calif.) at a Senate Judiciary Committee hearing on March 14. (Aaron P. Bernstein/Bloomberg News)

The heads of the Senate Judiciary Committee are expanding their ongoing probe of Russian interference in the 2016 presidential election to the communications of two Trump campaign advisers who went on to work in the White House, according to a letter requesting records Thursday.

In a letter to Trump campaign lawyer Ben Ginsberg, Senate Judiciary Chairman Charles E. Grassley (R-Iowa) and ranking Democrat Dianne Feinstein (D-Calif.) requested the “responsive emails” of John Mashburn and Rick Dearborn.

Mashburn, who served as policy director for the Trump campaign, now works as deputy Cabinet secretary in the Trump administration, while Dearborn, who joined the campaign after several years as an aide to now-Attorney General Jeff Sessions, worked as the president’s deputy chief of staff until announcing his resignation in December.

Dearborn’s name has surfaced in conjunction with various attempts by Russians to meet with members of the Trump campaign. According to the New York Times, Dearborn was the recipient of an email called “Kremlin Connection” from conservative activist Paul Erickson, telling him that Russian President Vladi­mir Putin wanted to meet with Donald Trump before the election and would try to make contact at an upcoming convention of the National Rifle Association.

Dearborn also appears to have turned down a request from Russian gun rights activist Maria Butina to discuss the persecution of Christians after Trump’s son-in-law and adviser, Jared Kushner, rejected the suggestion.

Dearborn and Mashburn were also the recipients of phone calls made by campaign foreign policy aide J.D. Gordon as delegates to the Republican National Convention were debating what the party’s platform on Ukraine should be. The party ultimately elected to adopt language that endorsed providing Ukraine with assistance, but not weapons, to defend itself from Russian aggression.

“We are writing today because we believe information obtained in a recent Committee interview warrants expanding those searches to two additional custodians who were not included in the original effort,” Grassley and Feinstein wrote in their letter, noting that the emails would “help ensure the Campaign’s production fully responds to the categories of documents sought in the Committee’s initial letter.”

The Judiciary Committee initially requested documents from the Trump campaign on July 19, and in the months since it has received over 28,000 pages of documents and letters, covering the 21 campaign officials whose communications they requested and reflecting what was produced by using over 300 search terms.

“The Committee has determined that obtaining [Mashburn’s and Dearborn’s] responsive emails, or at least ensuring that adequate searches have been conducted that would cover these communications, is necessary for its investigative work,” Grassley and Feinstein wrote, giving Ginsberg a deadline of April 12 to provide the requested materials.

Ginsberg did not immediately respond to an email requesting comment.

The Judiciary Committee is one of three congressional panels that has been probing questions related to the 2016 election and the Trump campaign’s alleged ties to Russian officials.

As part of that investigation, the Judiciary Committee announced last month that it would be releasing the transcripts of interviews conducted with the participants in a June 2016 meeting in Trump Tower with a Russian lawyer, including a transcript of the panel’s interview with President Trump’s son Donald Trump Jr. Two other Trump campaign officials, Kushner and former campaign manager Paul Manafort, never sat with the committee for an interview.

A spokesman for the committee said the request for materials from Dearborn and Mashburn is unrelated to the production of those transcripts.