Story highlights Rep. Adam Schiff said Intelligence Chairman Devin Nunes is blocking key witnesses

Schiff's remarks underscore the political impasse in the House Russia investigation

(CNN) Rep. Adam Schiff, the ranking Democrat on the House Intelligence Committee, said Thursday that Republicans are blocking dozens of witnesses from being interviewed in the Russia probe, including some who were aware of the 2016 Trump Tower meeting where Donald Trump Jr. was promised dirt on the Clinton campaign.

In a briefing with reporters, Schiff accused Republicans of blocking the inquiry from moving ahead, leaving key questions unanswered. He said that the House Intelligence Committee has spoken to 56 witnesses, fewer than half of the number in the Senate Intelligence Committee. Among the witnesses he wants to interview: Ivanka Trump, plus he says the panel should consider issuing a subpoena to Trump Jr. for information he withheld during his classified hearing last year.

"We can't do a thorough and credible investigation unless we obtain these documents -- unless we talk to these witnesses," Schiff said.

Schiff placed the blame largely on Republican Devin Nunes, saying the GOP chairman of the committee -- who stepped aside from running the investigation for much of last year -- has blocked dozens of requests for witnesses and subpoenas, with the support of House Speaker Paul Ryan.

The White House declined to comment on Schiff's accusations. Aides to Nunes and GOP Rep. Mike Conaway, who is running the Russia probe, did not respond to requests for comment. An attorney for Trump Jr couldn't be reached for comment, and an Ivanka Trump attorney had no immediate comment.

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