Despite a mountain of evidence supplied by cooperative diplomats — and a public admission and hasty retraction by acting White House chief of staff Mick Mulvaney — the uncertainty surrounding the hold on the aid has only deepened over time, according to interview transcripts released this week as part of the impeachment inquiry.

In fact, what has become increasingly clear is that only a small cadre of budget officials — and Trump himself — has the answers. And they have fought harder than anyone to spurn Democrats’ demands for testimony.

“We’re looking for ways to fill that in,” said Rep. Jamie Raskin (D-Md.), a member of the House Oversight and Reform Committee. “But if their refusal to testify keeps us from totally filling in the picture, we will fill it in with adverse inferences against the people who are refusing to testify.”…

If Democrats intend to make a public case that Trump attempted to shake down his Ukrainian counterpart, they will face pressure to address how the outright stonewall by Trump’s budget officials has impeded their effort to solve this mystery.