House Democrats expanded their impeachment inquiry Monday, subpoenaing documents from the Office of Management and Budget and the Pentagon. It’s part of an investigation into a whistleblower’s complaint that President Trump withheld military aid to Ukraine in exchange for a promise that its leaders would help Trump’s 2020 re-election campaign.

On Monday, Deputy Assistant Secretary of State George Kent failed to appear for a scheduled deposition with three House committees. Those committees were set to interview another key figure in the scandal today: Gordon Sondland, the U.S. ambassador to the European Union. But on Tuesday morning, the State Department said it had ordered Sondland not to testify to Congress. House Intelligence Committee chair Adam Schiff blasted Sondland’s canceled appearance as, “additional strong evidence of obstruction.”

Sondland is a wealthy hotelier and real estate magnate whom Trump tapped as U.S. ambassador to the European Union after he donated $1 million to Trump’s inaugural committee. Text messages reveal Sondland helped to mediate conversations between Ukrainian officials and Trump’s personal attorney, Rudy Giuliani, aimed at winning a promise by Ukraine’s president to investigate Joe Biden and his son and to look into a conspiracy theory that the 2016 hack of the Democratic National Committee’s email server was initiated in Ukraine. When the top U.S. diplomatic official in Ukraine texted, “Are we now saying that security assistance and White House meeting are conditioned on investigations?” Sondland texted back, “Call me,” later adding, “I suggest we stop the back and forth by text.”

This comes as The Washington Post reports that House Democrats are considering ways to conceal the identity of the whistleblower whose complaint spawned the impeachment probe. Democratic leaders are considering having the person testify by video stream, with their image and voice obscured, out of fear that Republicans would betray their identity to the White House. President Trump has accused the whistleblower of espionage and treason and suggested they should get the death penalty.