One of special counsel Robert Mueller's top prosecutors said there was a "notable" takeaway from U.S. Ambassador to the European Union Gordon Sondland's impeachment testimony on Wednesday.

Andrew Weissmann, a former Justice Department official who was known as Mueller's "pitbull" during the Russia investigation, suggested House Democrats should take a cue from Sondland's opening statement and seek out the emails, phone records, and other documents that he testified the White House and State Department have blocked him from accessing.

"The one thing that I think was notable in especially the opening statement was that you had the witness telling you there are documents out there — get them," Weissmann, now an NBC News legal analyst, said on MSNBC.

House Democrats subpoenaed Sondland last month after the State Department blocked his testimony. They also demanded documents related to Ukraine but have not received them.

Noting the Democrats are reluctant to commit to a protracted legal fight for the material, Weissmann emphasized that doing so would send a signal to upcoming impeachment witnesses.

"[Sondland] made a point of wrapping himself around the documents to corroborate what he was saying. And so there are obviously are good reasons why the Democrats are saying we don’t want to go down that road and we don’t want to get into a rabbit hole of the court system and how long that takes. I’m not sure at this point, with this witness saying they're there, why you don't do it," Weissmann said. "You may not get them in time. But there is a reason to do it even if you don’t get them in time, which is the witnesses who are coming up know that when they tell you a story, they need to worry about some day those documents are going to come out."

Sondland testified that President Trump should not have put a hold on security assistance while urging Ukrainian President Volodymyr Zelensky to open a pair of politically charged investigations. “I really regret that the Ukrainians were placed in that predicament,” Sondland told the House Intelligence Committee during a morning session of the impeachment hearing.

Sondland’s appearance follows other witnesses who forced him to retract his claim, during a closed-door deposition in October, that there was no quid pro quo in the Trump team’s engagement with Ukraine. Sondland, protesting Wednesday that the impeachment process was “less than fair,” said that he had forgotten key details because he has not been allowed by the State Department to review his phone records and other relevant files.

“In the absence of these materials, my memory has not been perfect,” Sondland said. “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.”

Republicans cast Sondland's testimony as being unreliable "speculation" because of his lack of documents.