The Senate Judiciary Committee will take a hard look at the “unmasking” of Trump campaign and transition officials by the Obama White House at a closed-door hearing Wednesday — pushing an underexplored aspect of the scandal over Russian meddling in the November presidential election.

Former National Security Adviser Susan E. Rice and other top officials from the Obama White House have come under suspicion that they inappropriately asked to identify — “unmask” — American citizens caught up in U.S. intelligence intercepts of foreign figures under surveillance in the Russia probe, apparently trying to find out whether Mr. Trump and his top aides were caught up in the investigation.

While Republicans have led the charge for a deeper investigation into the unmasking accusations, it was a key Democrat on the Judiciary Committee who raised the issue during a public committee hearing on Tuesday, asserting that lawmakers hope to explore it more deeply with FBI officials during a classified hearing Wednesday.

Sen. Sheldon Whitehouse, Rhode Island Democrat, told Carl Ghattas, executive assistant director of the FBI national security branch, that he specifically wants answers during the closed-door hearing about the leaks surrounding President Trump’s former national security adviser, Michael Flynn, who resigned over contacts with Russia.

“There is a specific allegation that has been floating around that the conversations between Michael Flynn and [Russian] Ambassador [Sergey] Kislyak came into the White House with Flynn masked,” Mr. Whitehouse told his fellow committee members. “And that a decision was made in the Obama White House to unmask him — which then led to leaks to the newspapers and so forth.”

Mr. Ghattas heads up the FBI’s operations and intelligence efforts involving national security matters. In March, Ms. Rice admitted to requesting the unmasking of some Americans redacted in raw intelligence reports but argued that the requests were part of her job and were not politically motivated.

Rep. Devin Nunes, California Republican and chairman of the House Permanent Select Committee on Intelligence, has been an outspoken proponent of focusing on the issue.

The House intelligence panel last month issued three subpoenas to intelligence agencies for records relating to alleged unmasking requests made by senior Obama administration figures, including Ms. Rice, CIA Director John O. Brennan and U.S. Ambassador to the U.N. Samantha Power.

• Alex Swoyer contributed to this story.

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