"We believe that the murder was premeditated and the body was subsequently moved out of the consulate."
A Turkish official also told The Associated Press that detectives' "initial assessment" was that Jamal Khashoggi was killed at the consulate, without elaborating.
Saudi authorities early on Sunday called the allegation "baseless."
"If the reports of Jamal's murder are true, it is a monstrous and unfathomable act," the Washington Post's editorial page editor Fred Hiatt said in a statement. "Jamal was - or, as we hope, is - a committed, courageous journalist. He writes out of a sense of love for his country and deep faith in human dignity and freedom."
The Post cited one anonymous official who said investigators believe a 15-member team "came from Saudi Arabia." The official added: "It was a preplanned murder."
Khashoggi, a contributor to the Washington Post, went to the Saudi consulate in Istanbul on Tuesday to receive an official document for his marriage and has not appeared since.
His Turkish fiancee reported him missing after Khashoggi, who has lived in self-imposed exile in the United States since last year, did not emerge from the consulate.
Saudi Arabia had dismissed earlier speculation about an abduction, however Turkish police on Saturday widened an investigation into the journalist's whereabouts.
Saudi Crown Prince Mohammed bin Salman has launched a fierce crackdown on opponents and critics of his rule.
At least 11 journalists are currently being detained in Saudi Arabia and the kingdom ranks as one of the worst in the world for the media.
Bin Salman has also detained dozens of potential royal and business rivals and held them in Riyadh's Ritz Hotel, including world famous entrepreneur Prince Alwaleed bin Talal.
A recent BBC documentary also claimed that three princes were abducted by Saudi authorities from abroad before being hurried back to the kingdom, and have not been heard of since.