Sen. Ted Cruz (R-Texas) and former CIA director John Brennan traded barbs regarding the killing of Iran's top nuclear scientist, Mohsen Fakhrizadeh.

"This was a criminal act & highly reckless. It risks lethal retaliation & a new round of regional conflict," Brennan wrote on Twitter. "Iranian leaders would be wise to wait for the return of responsible American leadership on the global stage & to resist the urge to respond against perceived culprits."

"I do not know whether a foreign government authorized or carried out the murder of Fakhrizadeh," Brenna continued. "Such an act of state-sponsored terrorism would be a flagrant violation of international law & encourage more governments to carry out lethal attacks against foreign officials."

"These assassinations are far different than strikes against terrorist leaders & operatives of groups like al-Qaida & Islamic State, which are not sovereign states," he tweeted. "As illegitimate combatants under international law, they can be targeted in order to stop deadly terrorist attacks."

Cruz reacted by saying, "It's bizarre to see a former head of the CIA consistently side with Iranian zealots who chant 'Death to America.' And reflexively condemn Israel. Does Joe Biden agree?"

Brennan, who was director of the CIA between 2013 and 2017, retorted, "It is typical for you to mischaracterize my comment."

"Your lawless attitude & simple-minded approach to serious national security matters demonstrate that you are unworthy to represent the good people of Texas," Brennan said, besmirching the senator.

Cruz then hit back, by pointing out that Brennan voted for Gus Hall, a U.S. Communist Party candidate who only secured 0.07% of the vote in 1976.

"Gosh, I touched a nerve. Facts matter," Cruz tweeted. "FACT 1: you've admitted, in 1976 you voted for the Communist candidate for President because, I guess, Jimmy Carter wasn't liberal enough for you."

Then the Texas senator took aim at the Obama administration's cozy relationship with the Iranian government.

"FACT 2: you gave $150 Bn to Ayatollah Khamenei, knowing it would be used to kill Americans," Cruz said.

The two then exchanged blows about the Logan Act, which "forbids private citizens from engaging in unauthorized correspondence with foreign governments," and nobody has been indicted on the legislation since 1852.

"@tedcruz is now misrepresenting the Logan Act. Apparently, he is a poor Senator & a bad lawyer," Brennan wrote. "A private citizen publicly criticizing what could be a state-sponsored assassination of a government official & cautioning against retaliatory killing is called freedom of speech."

"In contrast, an individual tapped to be National Security Advisor (but not yet one) in an incoming Administration who privately contacts a foreign government official & advises how to conduct relations with the U.S. could be in violation of the Logan Act," Brennan said.

"My commitment to Israel's security & my efforts to counter Iran's malign activities are well documented throughout my 33 years of national security work serving Republican & Democratic Administrations," Brennan stated, then attacked Cruz. "Aside from his tiresome rhetoric, what has Senator Cruz ever done?"

Cruz fired back, "Ok, let's apply your proffered standard: Ben Rhodes has publicly stated, 'foreign leaders are already having phone calls with Joe Biden, talking about the agenda they're going to pursue on Jan. 20.' Does that mean you think Joe Biden should be prosecuted under the Logan Act?"

"The only coherent answer is NO, neither Biden nor Gen. Flynn violated the Logan Act bc it's unconstitutional—which is why nobody has been convicted of violating it in the history of the US," Cruz contended. "But you want to use it as a partisan cudgel, a weapon to attack your political opponents."

Fakhrizadeh, who was considered the top architect of Iran's nuclear program, was gunned down in a road ambush in broad daylight on Friday. The black sedan the scientist was traveling in was riddled with bullets.

Iranian Foreign Minister Mohammad Javad Zarif said the assassination was "an act of state terror," there were "serious indications" of Israeli involvement.

On Saturday, Iran's supreme leader promised to retaliate for the killing of Fakhrizadeh. Ayatollah Ali Khamenei demanded that Iranian officials make a pledge into "pursuing this crime and punishing its perpetrators and those who commanded it."

Iran's President Hassan Rouhani vowed that the Islamic republic "will respond in time to the assassination of Martyr Fakhrizadeh."

"Once again, the evil hands of Global Arrogance and the Zionist mercenaries were stained with the blood of an Iranian son," Rouhani added.