(CNN) House Minority Leader Nancy Pelosi sent a letter Thursday to House Speaker Paul Ryan urging him not to allow the House Russia investigation to be shut down.

Pelosi's letter says Democrats are "deeply concerned by the Majority's efforts to curtail the House Intelligence Committee investigation and its overall failure to address Russia's meddling in the 2016 election."

"Nothing less than America's democracy and national security are at stake, and therefore we must fully investigate Russia's assault on our election systems to prevent future foreign attacks," the California Democrat wrote. "We expect that you will take urgent action to ensure this investigation can continue, and justice can be pursued, unhindered."

Rep. Adam Schiff of California, the top Democrat on the House Intelligence Committee, warned last week that he was concerned House Republicans might end the committee's Russia investigation at the end of the year, with no witnesses yet scheduled in 2018.

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