Obama-era CIA Director John Brennan on Friday lashed out at the killing of the head of Iran’s nuclear program, Mohsen Fakhrizadeh, labeling his death a “criminal act.”

Iran announced that Fakhrizadeh was assassinated in Damavand Friday afternoon. Images of the scene show two vehicles, one with extensive fire damage and the other riddled with bullet holes. Iranian Foreign Minister Mohammad Javad Zarif appeared to blame Israel for the nuclear expert’s killing.

“This was a criminal act & highly reckless. It risks lethal retaliation & a new round of regional conflict,” Brennan wrote of Fakhrizadeh’s death. “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 the [sic] murder of Fakhrizadeh,” he 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. As illegitimate combatants under international law, they can be targeted in order to stop deadly terrorist attacks,” he concluded.

Brennan is an ardent defender of the Joint Comprehensive Plan of Action, known commonly as the Iran nuclear agreement, and harshly criticized President Donald Trump for scrapping the deal in 2018.

“Donald Trump simultaneously lied about the Iranian nuclear deal, undermined global confidence in US commitments, alienated our closest allies, strengthened Iranian hawks, & gave North Korea more reason to keep its nukes. This madness is a danger to our national security,” Brennan said after the president announced the U.S.’s removal from the deal.

In January, the U.S. killed Gen. Qassem Soleimani, the head of Iran’s elite Quds Force, in an airstrike at Baghdad’s international airport. At the time, the Pentagon said it took out Soleimani because he “was actively developing plans to attack American diplomats and service members in Iraq and throughout the region.”