House Intelligence Committee Chairman Adam B. Schiff on Tuesday bemoaned the State Department’s decision to block U.S. Ambassador to the European Union Gordon Sondland from testifying before the committee during its impeachment inquiry, calling the decision “obstruction”.

Sondland’s testimony, the California Democrat said, would have been “deeply relevant” to the inquiry into whether President Donald Trump used his office to coerce the Ukrainian government into investigating a chief political rival ahead of his 2020 re-election bid.

Schiff called the administration’s decision to block Sondland’s testimony “obstruction of a co-equal branch of government.”

Later, Schiff, along with the chairmen of the Foreign Affairs and Oversight committees, said in a joint statement that they consider the State Department’s blocking Sonland from testifying before the committees and turning over documents to be obstruction of the House’s impeachment inquiry.

“We will be issuing subpoena to Ambassador Sondland for both his testimony and documents,” the chairmen said.

While Ukraine is not a member of the European Union, Schiff said Sondland had been in contact with Ukrainian officials regarding matters directly pertinent to the investigation, including whether Trump decided to withhold military aid to Ukraine unless Kiev investigated Hunter Biden, the son of 2020 presidential hopeful Joseph R. Biden Jr.

“By preventing us from hearing these witnesses and obtaining these documents,” Schiff said, “the president and secretary of State are preventing us from getting the facts needed to protect the nation’s security.”

Text conversations turned over to Congress last week by Kurt Volker, the former U.S. envoy to Ukraine, indicate that Sondland had discussed with government officials events related to the impeachment inquiry, Schiff said.

“We know from those text messages that diplomatic personnel raised a concern with [Sondland] that military assistance was being withheld to secure help from Ukraine in the president’s reelection campaign,” Schiff said, noting that Sondland had been in touch with the president and a U.S. senator. “We know that Ambassador Sondland had at least one discussion with a fellow diplomat on that very subject of why military assistance was being withheld.”

Even though the administration blocked his testimony, Sondland was prepared and willing to field lawmakers’ questions.

“Ambassador Sondland is profoundly disappointed that he will not be able to testify today,” Sondland’s lawyer Robert Luskin of Paul Hastings LLP said in a statement. “Ambassador Sondland traveled to Washington from Brussels in order to prepare for his testimony and to be available to answer the Committee’s questions. Arrangements had already been made with Joint Committee staff regarding the logistics of his testimony.”

In a Tuesday morning tweet, Trump defended his administration’s decision, saying Sondland would have been “testifying before a totally compromised kangaroo court.”

House Republicans echoed the president’s tweet.

Ohio Republican Jim Jordan supported the decision to block Sondland’s testimony, saying that Schiff is conducting an “unfair and partisan” investigation.

Jordan pointed to Schiff’s handling of Volker’s closed-door congressional testimony last week as a reason why he supports the decision to block Sondland from appearing.

“When you’re going to have a chairman release certain parts of what was said in a closed-door interview and not release the entire transcript and give it context, you can see why they didn’t want to subject Ambassador Sondland to the same kind of treatment,” Jordan said.

Later this week, Marie Yovanovitch, the former U.S. ambassador to Ukraine, is scheduled for a closed-door deposition with House investigators.

Lindsey McPherson, John T. Bennett and Niels Lesniewski contributed to this report.