WARSAW — The nearly disastrous plunge of the Serbian president’s plane Friday was the fault of the co-pilot, who spilled coffee in the cockpit and accidentally activated an emergency switch, the Serbian Civil Aviation Directorate said in a report published Tuesday.

Initially, Serbian officials said that the aircraft, carrying President Tomislav Nikolic and nine others to a meeting with Pope Francis at the Vatican, had been sent into a plunge over the Adriatic Sea after one of its three engines failed at an altitude of about 33,000 feet.

But an investigation by the Civil Aviation Directorate found that the dive occurred after the co-pilot, Bojan Zoric, “spilled coffee on the instrument panel” and “accidentally activated the emergency slot extension” as he was trying to clean it up.

This led to a brief shutdown of the third engine, the report said. After about a minute, in which the plane fell about a mile, the captain regained control and the aircraft returned to Belgrade. Mr. Nikolic had to postpone his meeting with the pope.