A Senate aide said the Iran briefing was scheduled for the Senate Foreign Relations Committee. It was supposed to be given by senior State Department officials, including Iran special envoy Brian Hook and David Schenker, the assistant secretary of State for the Middle East.

"We know they will vaguely claim it was a logistical issue, even though this briefing had been already locked into the schedule for days," the Senate aide said.

Press aides to Secretary of State Mike Pompeo did not immediately respond to a request for comment on Wednesday.

The canceled briefing on embassy security was originally going to focus on the situation in the African country of Burundi. It was supposed to be held for lawmakers and staff of various national security-focused committees and House and Senate leadership, the House aide said.

At congressional staff’s request, the topic was broadened to cover embassy security in the wake of the U.S. drone strike that killed Iranian general Qassem Soleimani earlier this month, the House aide said.

The Trump administration has given varying explanations for why it decided to kill Soleimani, at times claiming the Iranian general was planning “imminent” attacks on Americans.

Trump has said he believes that Soleimani may have been plotting attacks on as many as four U.S. embassies. But Defense Secretary Mark Esper said on Sunday that he hadn't seen intelligence saying so.

Congressional staffers had planned to ask questions about the shifting explanations and wanted to receive a global threat assessment for U.S. personnel. The State Department agreed to provide the embassy security briefing, then said Tuesday that it would not do so, the House aide said.

Pompeo has already been the target of ire from House Democrats who asked him to appear for a public hearing earlier this week. He did not appear, instead traveling to California to deliver speeches, including an address about the U.S. relationship with Iran at Stanford University's Hoover Institute.

On Tuesday, House Democrats released additional information they’d recently obtained from Lev Parnas, who worked closely with Giuliani on his efforts to dig up damaging information on former vice president Joe Biden in Ukraine.

Some of the material, including text messages, indicated that Republican congressional candidate Robert Hyde was working with Parnas to track Marie Yovanovitch, the U.S. ambassador to Ukraine at the time. It's not clear whether Giuliani knew of Hyde's activities, and Hyde has dismissed questions and suggested to reporters that he was making idle talk.

At Trump’s insistence, Pompeo recalled Yovanovitch from Ukraine in May as Giuliani circulated unsubstantiated reports claiming the veteran ambassador was acting corruptly.

The president described Yovanovitch as “bad news” in a July 25 phone call with Ukraine’s president, saying said she was “going to go through some things.”

Yovanovitch testified in Trump’s impeachment inquiry last year, and said that she felt frightened when she read the rough transcript of Trump's call.

Yovanovitch’s lawyer, Lawrence Robbins, says investigators should look into exactly what happened in terms of the surveillance.

“Needless to say, the notion that American citizens and others were monitoring Ambassador Yovanovitch’s movements for unknown purposes is disturbing,” Robbins said in a statement. “We trust that the appropriate authorities will conduct an investigation to determine what happened.”

The revelations has also troubled serving and former U.S. diplomats, some of whom were demanding to know what the State Department’s diplomatic security division did to protect Yovanovitch, and what, if anything, Pompeo knew about it.

