Iranian officials and military leaders have vowed revenge after a nuclear scientist was assassinated in Tehran.

Mohsen Fakhrizadeh was killed by a remote-controlled electronic device when riding in a car, according to Iran’s Secretary of Supreme National Security Council Ali Shamkhani. Earlier media reports have suggested there were up to a dozen gunmen involved in the attack. More recent reports suggest that a gun was attached to a car that fired on Fakhrizadeh’s vehicle. Many of the details about the attack remain murky, but Shamkhani said they know who “designed” the attack.

Iran’s Intelligence Minister Mahmoud Alavi said they have “many clues” as to who was behind the attack. Due to the “security aspect” of the attack, he said, officials cannot talk about the details until all dimensions of their investigations are pursued.

Hossein Salami, commander of Iran’s Islamic Revolutionary Guard Corps (IRGC), vowed revenge for the attack on Fakhrizadeh. “Enemies should be awaiting our revenge,” Salami said at Fakhrizadeh’s funeral. “The time, place and quality of the operation we will determine.” He said “terrorists want to suspend” Iran’s progress, but in the last 40 years since the founding of the Islamic Republic, over 17,000 people have been victims of terrorism and Iran’s progress has not been slowed.