ISTANBUL — Turkish officials on Wednesday announced the indictments of 20 Saudi nationals on charges of murder and incitement to murder in the killing of the dissident writer Jamal Khashoggi, concluding their investigation into the case.

In a statement, the Istanbul prosecutor’s office said the indictments, which have yet to be made public, would show the attack had been planned. It said Mr. Khashoggi had been strangled and dismembered in a planned murder inside the Saudi Consulate in Istanbul on Oct. 2, 2018. The same conclusion had been reached by American intelligence.

The prosecutor’s statement said that arrest warrants had been issued for the 20 men and “red notices” — detention requests to the world’s governments — issued via Interpol for all of them. Documents have also been prepared to request those in the indictment be handed over to Turkey.

But Saudi Arabia has not granted Turkish investigators access to the accused in Saudi Arabia, and is unlikely to cooperate in any extradition. The case is also unlikely to come to trial since none of the men are in the country and Turkish law demands the presence of the defendants for a trial.