(CNN) -- Authorities in Italy have called off the search for two Americans who went missing during a gas balloon race.
The search ended Monday afternoon and will not resume, said Massimo Maccheroni of the Italian coastal guards. No balloon debris was found during more than four days of searching, he said.
A balloon carrying Richard Abruzzo of New Mexico and Carol Rymer-Davis of Colorado over the Adriatic Sea near Italy made a rapid descent during rough weather, conditions that would make their survival "unlikely," race organizers have said.
"This is very pessimistic information," said a statement from the race organization -- Coupe Aeronautique Gordon Bennett.
Race officials from the competition said they lost contact with the balloon as it was over the sea Wednesday morning.
Croatian boats searched Croatian waters, Maccheroni said.
Officials looked at transponder readings from air traffic control and made calculations from the altitude of the balloon, the time and its location, said flight director Don Cameron. The data showed the balloon had a "moderate descent rate initially which then increased into a high rate of descent, to around 50 miles per hour."
"It is the opinion of the Gordon Bennett 2010 Flight Control Team that the balloon appears to have suffered a sudden and unexpected failure," the statement read. "The cause of this tragedy is still being examined."
Two U.S. aircraft helped eight Italian vessels and aircraft in the search, which covered 3,600 square miles.
Abruzzo's wife, Nancy, said last week that she was hopeful.
"We've had other challenges before, but we've never lost contact for this long," she said. "But they are smart, strong and capable. And they are survivors."
Maccheroni listed three hypotheses on what happened: Lightning struck the balloon and it exploded; the balloon suffered a failure and went down quickly into the ocean; or the balloon dropped slowly, making survivability more likely.
The balloon was equipped with a satellite phone, VHF radios, a radar transponder and two mobile phones, but efforts to contact Abruzzo and Rymer-Davis were unsuccessful.
The balloon also had survival suits, life jackets and two single-person life rafts, according to Cameron.
"We have not found any safety device [lifeboat], and one can last only ... so long at sea without one," Maccheroni said.
CNN's Hada Messia contributed to this report