President Donald Trump and Senate Majority Leader Mitch McConnell (R-KY) are engaged in a cover-up by trying to prevent the Senate from holding a fair impeachment trial. They are attempting to block witnesses from testifying because it is clear that these witnesses will provide damning evidence against the president.

Most of what we know about these potential witnesses’ roles has come from testimony or press reports. In some cases, such as those of Secretary of State Mike Pompeo and Acting Chief of Staff Mick Mulvaney, it also comes from public statements they themselves have made.

Pompeo, Mulvaney, former national security adviser John Bolton, and Vice President Mike Pence can testify to the president’s direct involvement in the plot to extort Ukraine. The other witnesses, who played roles in implementing the White House’s scheme, can shed light on what they did and on whose orders they did it. Furthermore, the documents they possess could corroborate the allegations against Trump. It is vital that Congress and the American public learn the truth—which means it is vital that the Senate hold a full trial and demand that the administration produce the witnesses and documents it is trying desperately to hide.

Trump’s defenders in Congress claim that they have heard no firsthand evidence of Trump’s abuses of power. That’s because Trump has stonewalled Congress, preventing those with firsthand knowledge from testifying or producing documents. To rectify that, here are 11 witnesses the Senate should call to testify in the impeachment trial and what they should be asked about.