Senior White House officials suspected that a colleague whom they viewed as a political partisan was ferrying documents about Ukraine in recent months to President Trump, effectively creating another back channel to him that could warp American policy, according to congressional testimony and interviews.

Colleagues grew alarmed after hearing that Mr. Trump had referred to Kashyap Patel, a National Security Council aide who figured prominently in Republicans’ efforts to undermine the Russia investigation, as one of his top Ukraine policy specialists and that the president wanted to discuss related documents with him, according to people briefed on the matter. Mr. Patel, who is known as Kash, is assigned to work on counterterrorism issues, not Ukraine policy.

The contents of the documents were not clear, nor was it clear how Mr. Trump got them. Typically, aides prepare policy briefings for presidents that several agencies sign off on in a highly controlled process. But Mr. Trump has adopted a much more freewheeling approach, taking in unverified information from sources both inside and outside the White House and seeking out and promoting assertions that fit his narrative.