The impeachment inquiry into Donald Trump’s Ukraine dealings that began last month is well underway now, despite the White House’s efforts to delegitimize the probe. State Department officials who have been instructed not to testify before Congress are coming forward.

One reason for this cooperation is lawmakers’ subpoena power.

The president’s lawyer wrote a letter on Oct. 8 denouncing the investigation and alleging that it is unconstitutional and unprecedented and warning Congress that the administration won’t cooperate. But the claims in the letter contradict the impeachment process described by constitutional law experts. And administration officials who fail to comply with a subpoena to testify do risk criminal and civil fines, including jail time.

That fact gave Marie Yovanovitch, the former ambassador to Ukraine, added justification for speaking to congressional investigators yesterday (Oct. 11). EU ambassador George Sondland announced that next week he will testify, too. “Notwithstanding the State Department’s current direction to not testify, Ambassador Sondland will honor the Committees’ subpoena, and he looks forward to testifying on Thursday,” his office said in a statement yesterday. “Ambassador Sondland has at all times acted with integrity and in the interests of the United States. He has no agenda apart from answering the Committees’ questions fully and truthfully.”

Both Yovanovitch and Sondland were subpoenaed by congressional investigators. As the New York Times reported, Yovanovitch was subpoenaed “quietly” on Friday ahead of her scheduled testimony, creating a legal basis for her to disobey State Department orders not to testify. Trump had called Yovanovitch “bad news.” She says she was pushed out of her position because her efforts in Ukraine to fight corruption, a stated US policy, undermined the president’s personal interests. The ousted ambassador may well have wanted to testify and tell her side of the story of her ouster, but it’s subpoena power that required her to do so by law and thus protected her while she disobeyed State Department orders.

Yovanovitch, and indeed Sondland, have to obey lawmakers now. Subpoenas are issued by courts and government authorities to produce records or testimony. The power to subpoena has long been used by Congress and is supported by Supreme Court precedent. Although a subpoenaed party may negotiate the terms of their appearance or document production, and they often do, they are not expressly entitled to make a deal. And if they choose not to comply, authorities have several enforcement options.

Congress could pursue criminal contempt charges, asking the US attorney for DC in the Justice Department to bring a case. The maximum punishment for a misdemeanor federal contempt charge is a $100,000 fine and a year in jail. Representatives could also file a civil suit seeking a court’s power to enforce the subpoena. Or, members of Congress can call the Sergeant-at-Arms to detain the disobeying party.

None of these options is particularly appealing for investigators. The Justice Department, headed by Trump stalwart William Barr, is unlikely to pursue a criminal case on behalf of Democratic investigators of the president. A civil suit could take months or more to play out in the courts. And detaining someone via the Sergeant-at-Arms is a rare remedy that hasn’t been used since 1935, noted Margaret Taylor, a former Democratic Chief Counsel for the Senate Foreign Relations Committee, while speaking to PBS.

Yet another option exists, and this is the one that impeachment investigators are relying upon to punish failure to comply on Trump’s part. Stonewalling the investigation can be considered evidence of obstruction of Congress (pdf) and can justify articles of impeachment. One of the charges against Richard Nixon in his 1974 articles of impeachments, in fact, was that he instructed witnesses to give false testimony and to mislead investigators. Trump’s total refusal to comply is arguably even more egregious and may well become the basis of a separate article of impeachment should lawmakers ultimately decide to vote on impeachment.

When secretary of state Mike Pompeo objected to his officials testifying in an angry letter on Oct. 1, he pointed to procedural defects in the requests, most notably the fact that subpoenas weren’t issued for the witnesses, who were simply asked via a letter to appear. Investigators heard Pompeo and addressed his contentions by also issuing subpoenas in such cases. Now there is little that witnesses can do besides comply with their subpoenas or risk jail time and fines. Loyalty to Trump could come at a high price and isn’t guaranteed to advance his cause in any case.