Her inquiry does not amount to a formal, independent investigation of the kind the United Nations has ordered in the past, notably into the assassination of former Prime Minister Benazir Bhutto of Pakistan in 2007. But it is a first step that might lead to a some fuller United Nations examination of the case.

It was not clear whether the announcement would satisfy Turkey, which has called on the United Nations to launch a full independent, international investigation in the Khashoggi case.

Ms. Callamard’s investigation will begin with a trip next week to Turkey, aided by Helena Kennedy, a British lawyer, member of the House of Lords and former official at the University of Oxford; and Duarte Nuno Vieira, a leading expert in forensic medicine and professor of medicine at the University of Coimbra in Portugal.

American intelligence agencies have concluded that Prince Mohammed bin Salman, Saudi Arabia’s 33-year-old crown prince and de facto ruler, personally ordered the assassination of Mr. Khashoggi, a Virginia resident who was killed and dismembered, and whose remains Turkish investigators have yet to find. After a briefing on the issue by the director of the Central Intelligence Agency, the Senate passed a resolution publicly holding the crown prince responsible.

Saudi Arabia has said it is conducting its own investigation into the killing, and Saudi officials have said their initial inquiry has indicated that the leader of a team of agents on the ground in Istanbul — not the crown prince — decided to kill Mr. Khashoggi. The kingdom has said it has arrested 21 people in connection with the killing and this month it reportedly began prosecuting 11 of them. The kingdom has said it is seeking the death penalty for five of those defendants but has not named any of them.