Missing balloonists Carol Rymer-Davis and Richard Abruzzo, in an undated photo.

Italian authorities have ended the search for two missing Americans that disappeared during a European hot air balloon race.

The search was called off Monday afternoon, and it is not expected to be resumed. After four days of searching, no signs of the balloonists have been found.

Richard Abruzzo, from New Mexico, and Carol Rymer-Davis, from Colorado, were flying over the Adriatic Sea, off the coast of Italy, when bad weather forced their balloon into a quick descent. Race organizers believe the survival of the two is unlikely given the circumstances.

Contact was lost with the balloon on Wednesday morning. Transponder readings indicate that the balloon quickly went from a moderate descent to a rapid descent, dropping at approximately 50 miles per hour. It’s thought that there was a sudden failure in the balloon. No cause has been determined at this time.

A search by the Italian Coast Guard and two U.S. aircraft has spanned four days and covered 3600 square miles of water. but has produced no results. The balloon included a satellite phone, VHF radio, radio transponder and two mobile phones, yet no contact has been made, and no safety devices have been found.

The race, which included 20 hot air balloons, started last weekend in England. Balloonists were competing to fly the farthest from the starting point. The Rymer-Davis/Abruzzo team won a similar balloon race in 2004, which started in France.