Two intelligence officers at Iran's consulate in Turkey instigated the killing last November of an Iranian dissident in Istanbul who criticised the Islamic Republic's political and military leaders, two senior Turkish officials told Reuters.
The accusation is likely to strain ties between Turkey and Iran, two regional powers which had grown closer under the government of Turkish President Tayyip Erdogan.
Masoud Molavi Vardanjani was shot dead on an Istanbul street on Nov. 14, 2019, a little over a year after the Turkish officials say he left Iran.
A police report into the killing, published two weeks ago, said Vardanjani had an "unusual profile". It said he worked in cyber security at Iran's defence ministry and had become a vocal critic of the Iranian authorities.
According to the report, Vardanjani had posted a message on social media targeting Iran's elite Revolutionary Guards in August, three months before he was shot dead.
"I will root out the corrupt mafia commanders," the post said. "Pray that they don't kill me before I do this."
Reuters was unable to independently confirm either Vardanjani's position at the Iranian defence ministry or his social media posts.
No one at the Iranian embassy and consulate in Turkey responded to calls on Friday seeking comment on Vardanjani's background or death. Asked about possible Iranian government involvement in the killing, a spokeswoman for Istanbul's police said the investigation was continuing and declined to comment further.
A week after the killing, US Secretary of State Mike Pompeo described it as "another tragic example in a long string of suspected Iran-backed assassination attempts" of Iranian dissidents. He did not elaborate further.
Turkish authorities did not publicly accuse the Iranian government of involvement at the time. But the two senior Turkish officials said that the Turkish government would now raise Vardanjani's killing with Iran and one of them said that Turkish prosecutors were also following the case.
The suspected gunman and several other suspects, including Turks and Iranians detained in the weeks after the killing, told authorities they had acted on orders from two intelligence officers at the Iranian consulate, the first official said.
"It was reflected in the testimonies of the arrested suspects that these two Iranians, carrying diplomatic passports, had given the order for the assassination," he said, identifying the two men by their first names and initials.
The second Turkish official said evidence including the suspects' statements suggested "Iranian nationals played a serious role in both instigating and coordinating" the killing.
Both of the Turkish officials said Ankara would soon deliver a formal response to Iran over Vardanjani's killing and the role they said was played by officials with diplomatic passports.
Vardanjani was on the radar of the Iranian authorities.
Two Iranian security sources said he had defied a warning from the Revolutionary Guards not to cooperate with Turkish firms on drone projects, without giving details. They said he had also approached the United States and European states to work for them, although Reuters could not corroborate this.
One of the Iranian sources said he had published documents online that he had either hacked or obtained from contacts in Iran and had ignored requests to contact the Iranian embassy in Ankara, instead meeting Americans and an Israeli diplomat. The source gave no details on the documents or his meetings.
The second Iranian source also said that Vardanjani had been warned about his contacts with foreign diplomats.
The second Turkish official compared Vardanjani's death to the October 2018 murder of Saudi journalist Jamal Khashoggi by a team of Saudi agents inside Saudi Arabia's Istanbul consulate.
Erdogan has said Khashoggi's killing was ordered at the "highest levels" of the Saudi government. Crown Prince Mohammed bin Salman has denied ordering the killing but said he bore ultimate responsibility as the kingdom's de facto leader.
On Wednesday, the Istanbul prosecutor's office said it had indicted 20 people, including a former aide to Prince Mohammed over Khashoggi's killing.
Relations between Turkey and Iran have been tested by the civil war in Syria, where they back opposing sides.
Turkey has been particularly angered by the role of Iranian-backed fighters in a Syrian government offensive against rebels backed by Turkey in Idlib, a Syrian province just over Turkey's southern border, launched not long after Vardanjani's killing.
A joint investigation by Istanbul police and Turkish intelligence reviewed more than 320 hours of footage, searched 49 premises and spoke to 185 people, the police report said.
Video footage broadcast on Turkish television after Vardanjani's killing showed a gunman running past two men as they walked in central Istanbul's Sisli neighbourhood at 10 pm on Nov. 14 last year. The gunman fired several shots at one of them, who fell to the ground while his companion took cover.
The Turkish officials said the companion walking with Vardanjani had struck up a friendship with him after he arrived in Istanbul from Tehran in June 2018 and had passed information about him to Iranian intelligence.
The morning before the killing, the companion, whom the police report and Turkish officials say was named Ali Esfanjani, went to the Iranian consulate. He later met the gunman to discuss details of the operation, the officials said.
The police report describes Esfanjani as the leader of the team that carried out Vardanjani's killing.
Esfanjani was spirited across the border into Iran three days later by an Iranian smuggler, the first Turkish official said, showing a copy of a bus ticket he had used under a fake name to get to Turkey's eastern border region of Agri.
Reuters could not confirm Esfanjani's whereabouts.