The conclusion was also reported by Asli Aydintasbas, a senior fellow with the European Council on Foreign Relations in Turkey, who was briefed on the findings by a senior Turkish official.

“I am afraid government officials now believe he is dead,” Ms. Aydintasbas said.

Officials do not have hard evidence, she added, but have nevertheless concluded Mr. Khashoggi is dead.

Turan Kislakci, the head of Turkish Arab Media Association, and also a friend of Mr. Khashoggi, told The Times that Turkish officials had called him and confirmed the death

“They confirmed two things, he was killed and his body was dismembered,” Mr. Kislakci said.

Earlier on Saturday, Turkey’s semiofficial Anadolu news agency reported that 15 Saudi citizens, including Saudi diplomats, had arrived in Istanbul on two separate planes and were at the consulate around the same time as Mr. Khashoggi on Tuesday. They later left Turkey for the countries they came from, Anadolu reported.

Two of the sources with knowledge of the investigation said the 15 Saudis had arrived to silence Mr. Khashoggi, but that it was not clear if the plan had been to bring him back to Saudi Arabia alive, and something went wrong, or if the intention was to kill him there.

The Saudi consulate-general in Istanbul, Mohammad al-Otaibi, on Saturday invited Reuters to tour the consulate to show the news organization that Mr. Khashoggi was not there. “I would like to confirm that Jamal is not at the consulate nor in the Kingdom of Saudi Arabia, and the consulate and the embassy are working to search for him,” he said. “The idea of kidnapping a Saudi citizen by a diplomatic mission is something that should not be in the media.”

In an interview with The New York Times, Hatice Cengiz, the Turkish fiancé of Mr. Khashoggi, said she was following reports of his death but was waiting for Turkish officials to confirm it with her.

“I cannot think such an incident is acceptable to happen in Turkey,” Ms. Cengiz said.

“If such a thing had happened, Turkey would never keep silent, and Saudi Arabia would never attempt to do such a thing against Turkey,” Ms. Cengiz added. “Turkey would carry this to every kind of international arena, and it is not a country, not a government, that would swallow this. Saudi Arabia would not be able to answer this. Turkey would never accept this. This is impossible to happen in Turkish history.”