Sweden's rescue service reported that a private Cessna plane had crashed off the coast of Latvia on Sunday evening, after Nato scrambled jets to follow its erratic course.
The aircraft took off at 12.56pm from Jerez in southern Spain without a set destination, the FlightRadar24 website said.
The Austrian-registered Cessna 551 turned twice, at Paris and Cologne, before heading straight out over the Baltic, passing near the Swedish island of Gotland.
At 17.37pm it was listed on the flight tracker as rapidly losing speed and altitude.
"We've learned that the plane has crashed [in the ocean] north-west of the town of Ventspils in Latvia," a representative of Sweden's rescue service said. "It has disappeared from the radar."
German and Danish warplanes had earlier been sent to inspect the aircraft as it passed through their airspace, but were unable to make contact, Johan Wahlstrom of the Swedish Maritime Administration said.
"They could not see anyone in the cockpit," Mr Wahlstrom said.
German newspaper Bild reported that the plane was carrying the pilot, a man, a woman, and a person it described as a daughter, without giving sources for the information.
A Lithuanian air force helicopter was sent to the crash site at neighbouring Latvia's request, a Lithuanian air force representative said. Latvia said it had sent ships to the scene.
"Our ships are on the way to the position where the plane crash happened," said Liva Veita, spokeswoman for the Latvian Navy.
A Stena Line ferry travelling from Ventspils to Norvik in Sweden was also redirected to the crash site, the MarineTraffic website said.
It also showed a Swedish search and rescue helicopter and airplane at the site.
The company listed as the plane's owner in Austria’s aircraft register, GG Rent, could not immediately be reached for comment.
Fighter aircraft from the Nato Baltic Air Police mission at Amari airfield in Estonia earlier took off to follow the plane, the Lithuanian Air Force said.
