Flight director Don Cameron said that high rate of descent, if confirmed, leads him to be "very pessimistic" about the fate of veteran pilots Richard Abruzzo and Carol Rymer Davis.
Abruzzo and Davis were participating in the 54th Gordon Bennett Gas Balloon Race when contact was lost Wednesday morning in rough weather over the Adriatic Sea.
Race organizers said the balloon "appears to have suffered a sudden and unexpected failure."
"It's very bad news," Cameron said. "At this rate of descent to the surface, survival would be unlikely."
Cameron said he received information Friday from Zagreb's air traffic control indicating the balloon was at 5,300 feet (1,615 meters) and descended at a rate of 50 mph until 600 feet (180 meters).
Cameron stressed that the Croatian readings were from the outer limits of its radar zone, though, and said he hoped they were incorrect.
"It's the only shred of hope," he said, adding that he hoped to confirm the data with Italian air traffic controllers in Brindisi, on the other side of the Adriatic, on Saturday.
The Italian coast guard, the U.S. Navy and Croatian coastal aircraft crews have been scouring the area around Croatia's distant, uninhabited islet of Palagruza.
The Abruzzo name is synonymous with ballooning. Abruzzo is the son of famed balloonist Ben Abruzzo, who was in 1981 part of the first team to cross the Pacific Ocean by balloon, and who was killed in a small airplane crash in 1985.
The younger Abruzzo and Davis won the 2004 edition of the Gordon Bennett race and the 2003 America's Challenge gas race - one of Abruzzo's five victories in that race.
Abruzzo's wife, Nancy, was in Bari at coast guard headquarters on Friday monitoring the search effort. She said her husband had made a final radio transmission saying he was preparing to ditch in the sea.
"We have every reason to believe that with his final transmission to air traffic that he would have had enough, you know, an adequate amount of time to prepare for an emergency sea landing which, you know, they are very prepared for," Nancy Abruzzo said.
But the Italian coast guard was unaware of any final radio transmission, said spokesman Lt. Massimo Maccheroni. He said the coast guard merely received information about the last automatic signal the balloon communicated to the air traffic control center in Bridinsi before losing contact.
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Winfield reported from Rome. Associated Press reporter Mark Carlson contributed from Phoenix.
