Former CIA Director John Brennan visited Capitol Hill Tuesday after congressional Democrats invited him to give a briefing on the situation in Iran.

Brennan arrived on Capitol Hill on Tuesday morning and was slated to brief House Democrats during their private weekly caucus meeting. Democrats also invited former State Department official and top negotiator of the Iran nuclear deal, Wendy Sherman, to join Brennan in the briefing.

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Earlier this year, Brennan visited Capitol Hill to meet with House Majority Leader Steny Hoyer, D-Md., and other House Democrats to speak on national security issues, according to a source familiar with that meeting. Brennan, though, has had his security clearance revoked.

The Iran briefing comes on the sidelines of other closed-door briefings slated for Tuesday led by members of the Trump administration, for both Republican and Democratic lawmakers in the House and the Senate. Democrats are expected to attend both the Trump administration’s briefing and Brennan’s.

Tensions between the U.S. and Iran have risen in recent weeks after the Trump administration ordered warships and bombers to the Middle East earlier this month to counter threatened attacks against U.S. interests by Iran or Iranian-backed forces.

The U.S. also ordered nonessential staff out of its diplomatic posts in Iraq days after Secretary of State Mike Pompeo told Iraqi intelligence officials in Baghdad that the U.S. had picked up intelligence that Iran is threatening American interests in the Middle East.

On Sunday, a rocket landed less than a mile from the U.S. Embassy in Baghdad’s heavily fortified Green Zone, the first such attack since September. An Iraqi military spokesman told reporters that the rocket appeared to have been fired from east Baghdad, which is home to several Iran-backed Shiite militias.

“If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!” Trump tweeted Sunday.

Meanwhile, Brennan has been at the heart of the ongoing controversy about the origins of the Russia investigation.

Last week, Fox News learned that a late-2016 email chain indicated that then-FBI Director James Comey told FBI subordinates that Brennan insisted the unverified and salacious anti-Trump dossier be included in the intelligence community assessment on Russian interference, known as the ICA.

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Fox News was told that the email chain, which is not yet public, referred to the dossier as “crown material,” but it was not clear why this apparent code was used.

Another former CIA official, though, said in a statement to Fox News that it was Comey’s recommendation that the Steele Dossier be included in the intelligence report.

The controversy comes as Attorney General Bill Barr appointed U.S. attorney from Connecticut, John Durham, to lead an investigation into the origins of the Russia probe.

Durham, known as a “hard-charging, bulldog” prosecutor, according to a source, will focus on the period before Nov. 7, 2016—including the use and assignments of FBI informants, as well as alleged improper issuance of Foreign Intelligence Surveillance Act (FISA) warrants. Durham was asked to help Barr to "ensure that intelligence collection activities by the U.S. Government related to the Trump 2016 Presidential Campaign were lawful and appropriate."

A source also told Fox News that Barr is working "collaboratively" on the investigation with FBI Director Chris Wray, CIA Director Gina Haspel, and Director of National Intelligence Dan Coats, and that Durham is also working directly with Justice Department Inspector General Michael Horowitz, who is currently reviewing allegations of misconduct in issuance of FISA warrants, and the role of FBI informants during the early stages of the investigation.

Fox News’ Samuel Chamberlain, Gregg Re, and The Associated Press contributed to this report.