House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) on Wednesday accused President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE and Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE of a “concerted” effort to obstruct the House impeachment investigation and warned them it could be used as evidence to draft an article of impeachment.

“They do so at their own peril,” Schiff said at the conclusion of his opening remarks on the fourth day of open hearings in the House impeachment inquiry, noting that the third article of impeachment against Nixon accused him of refusing to obey subpoenas from Congress.

In issuing his warning, Schiff seized on the opening statement of Gordon Sondland Gordon SondlandGOP chairman vows to protect whistleblowers following Vindman retirement over 'bullying' Top Democrat slams Trump's new EU envoy: Not 'a political donor's part-time job' Trump names new EU envoy, filling post left vacant by impeachment witness Sondland MORE, the U.S. ambassador to the European Union testifying in the impeachment inquiry on Wednesday. Sondland said the State Department and White House did not provide him with phone records, emails or other documents that could have helped him reconstruct his memory of the events at the center of the investigation into the administration’s dealings with Ukraine.

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“My lawyers and I have made multiple requests to the State Department and the White House for these materials. Yet, these materials were not provided to me. They have also refused to share these materials with this Committee,” said Sondland, who was testifying under subpoena.

“These documents are not classified and, in fairness, should have been made available. In the absence of these materials, my memory has not been perfect. And I have no doubt that a more fair, open, and orderly process of allowing me to read the State Department records would have made this process more transparent,” Sondland continued.

Sondland later said that the White House has shared with his attorneys certain "call dates and times," including those related to a July 26 call he had with the president, but that the White House has not shared with him any readouts of the call.

Schiff also seized on excerpts of Sondland’s opening statement as implicating leaders in the White House and State Department in an effort to condition a White House meeting on Ukraine launching investigations into 2016 election interference and a Ukrainian gas company, Burisma, that employed Hunter Biden on its board.

Sondland in his opening statement testified that an effort to use a White House meeting between Trump and Ukrainian President Volodymyr Zelensky to press Ukraine for investigations was widely known within the administration and that he kept those at the State Department and within the White House updated of his efforts.

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"Everyone was in the loop. It was no secret. Everyone was informed via email on July 19, days before the Presidential call. As I communicated to the team, I told President Zelensky in advance that assurances to ‘run a fully transparent investigation’ and ‘turn over every stone’ were necessary in his call with President Trump,” Sondland’s opening statement reads, quoting from an email he sent to Pompeo, acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE and others on July 19.

“We have not received a single document from the State Department as Ambassador Sondland’s opening statement will make clear, those documents will bear directly on this investigation and this impeachment inquiry,” Schiff said during his opening statement Wednesday.

Sondland also said he raised concerns at a meeting with Vice President Pence in September that security assistance to Ukraine was being held up as officials pushed for investigations sought by Trump and his personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE. Sondland later said that Pence nodded when he registered those concerns.

“The knowledge of this scheme was far and wide and including, among others, Secretary of State Pompeo and the Vice President,” Schiff continued.

Pence’s chief of staff, however, said later that the meeting Sondland described “never happened.”

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“The Vice President never had a conversation with Gordon Sondland about investigating the Bidens, Burisma, or the conditional release of financial aid to Ukraine based upon potential investigations,” Marc Short said in a statement.

“Ambassador Gordon Sondland was never alone with Vice President Pence on the September 1 trip to Poland. This alleged discussion recalled by Ambassador Sondland never happened,” Short said.

House investigators have unsuccessfully subpoenaed the White House, State Department and other agencies for documents in connection with the inquiry. The White House has refused to cooperate with the inquiry, describing it as an illegitimate effort by Democrats to overturn the results of the 2016 presidential election.

A number of officials — mostly career officials and not political appointees —have complied with subpoenas for testimony despite objections by the White House; other key witnesses, like Mulvaney, have evaded subpoenas for their testimony.

Schiff has previously warned that Democrats would use any failure to comply with congressional subpoenas as evidence of obstruction.

--This report was updated at 11:37 a.m.