A private plane crashed into the sea after making bizarre changes to its route and not replying to air-traffic control

• Four people were lost at sea as a plane crashed after veering off course and dropping from contact.
• Several military aircraft were scrambled to follow the jet, whose cockpit appeared empty.

A private Cessna jet crashed into the Baltic Sea after becoming unresponsive to air traffic control and taking an unplanned route, according to multiple reports.

No survivors had been found as of Monday. Those aboard were identified in German and Swedish media as 72-year-old German businessman Peter Griesemann, who was flying, as well as his wife Juliane, his daughter Lisa, and a male friend.

It was unclear what happened to send the plane plunging into Latvian waters on Sunday, hundreds of miles from its planned destination.

Nobody was visible in the cockpit or the cabin when military jets were scrambled to investigate while the Cessna was still airborne, a Swedish official told Insider.

The Cessna 551 set off from Jerez, Spain, at 2.56 p.m. local time on Sunday, and was bound for Cologne, Germany, according to flight-tracking site FlightRadar.

But soon after takeoff, the jet reported cabin-pressure problems and broke off contact with air traffic control soon after leaving Spanish airspace, German newspaper Bild reported. (Bild shares a parent company, Axel Springer, with Insider.)

German, Danish and Swedish military aircraft followed the Cessna as it passed erratically through their airspace, said Lars Antonsson, a spokesperson for the Swedish Maritime Administration, in an exchange with Insider.

The jet was followed into Swedish airspace by a Coast Guard helicopter, as well as two Swedish fighter jets, Antonsson said. Just as they had been told by their German and Danish counterparts, the Swedish pilots couldn't see anyone at the Cessna's controls, nor in the cabin, Antonsson said.

Some time before 8 p.m. local time, the jet began to lose altitude, and took a spiral path into Latvian waters, just off Ventspils, as it ran out of fuel, per Bild.

Aviation-safety expert Hans Kjäll told Swedish news agency TT that rapid cabin depressurization could have led the passengers and pilot to lose consciousness, according to the German state news service Deutsche Welle.

As the jet took its unexpected route, thousands watched its progress on a flight-tracking site, as investigative reporter Scott Stedman said on Twitter.

Latvia began search-and-rescue efforts, with no people found as of midnight Sunday, Antonsson told Insider. Only wreckage and an oil slick was visible as of early Monday, German newspaper Express reported.

A Cessna 551 is a type of small corporate jet with a capacity of up to 10 seats, according to aviation site Airliners.

Griesemann was the founder of Griesemann group, an engineering company, per an archived version of the company website.

The website, active as of late Sunday local time, was offline as of Monday. The company did not immediately respond to Insider's request for comment.

Griesemann was also known as the honorary president of a popular Cologne festival, the Blue Sparks Carnival, according to Bild. Its founder confirmed Griesemann's death in a statement to German news site Report-K.