Italy's ambassador in Venezuela said there are still hopes of finding a plane that disappeared carrying the CEO of Italian fashion firm Missoni, but the chances of finding survivors decrease with each passing day.

Venezuelan officials said on Wednesday that more than 400 people in boats, planes and helicopters were searching for a sixth day for the plane, which disappeared on Friday off the resort islands of Los Roques.

"We have to maintain hope up to the last moment," Italian Ambassador Paolo Serpi told The Associated Press in an interview on Tuesday. "We are working for this until the search is called off. Obviously, with each day that goes by, the hope diminishes."

The islands off Venezuela's coast are popular for pristine beaches and scuba diving along coral reefs. The 58-year-old Missoni had been vacationing in Los Roques along with his companion, Maurizia Castiglioni, and two Italian friends. The plane disappeared during a flight of about 95 miles (150 kilometers) from the islands to the Caracas airport.

Venezuelan authorities have said the twin-engine BN-2 Islander dropped off radar about 11 miles (18 kilometers) south of Los Roques.

Dan Cutrer, an aviation safety expert who used to be a search and rescue pilot for the U.S. Coast Guard, said that planes can drop off radar at various altitudes, and still continue to move for a time before going down. Cutrer said that searchers have to take into account that any wreckage would drift in the currents, and the area expands by the day.

"What the authorities are most likely doing at this point is they are using sophisticated computer programs, local knowledge of weather and they are plotting out search areas calculating where the drift would have taken an object the size of the aircraft or a person in a life raft or a person floating in the water," said Cutrer, an associate professor at Embry-Riddle Aeronautical University in Daytona Beach, Florida.

"The problem with maritime searches like that is every day that goes by, that search area grows exponentially because the winds and waves are still moving," Cutrer said.

He said that while such a small plane wouldn't have a cockpit data recorder as larger planes do, many such small planes carry an emergency locator transmitter that the pilot can turn on to send out a distress signal. But, he added, "they are subject to fail."

Serpi, who visited Los Roques and talked with officials leading the search, said weather conditions including choppy seas have added to difficulties for the searchers.

"It's a difficult situation, a very complicated natural environment," Serpi said, speaking at the Italian Embassy in Caracas.

Serpi acknowledged that it's possible the wreckage or bodies may never be found, pointing to the similar disappearance of a plane off Los Roques exactly four years earlier, on Jan. 4, 2008.

That plane, on a flight from Caracas to Los Roques, disappeared after crashing with 14 people aboard, including eight Italians, a Swiss man and five Venezuelans. The pilot had radioed to controllers that he was having engine trouble before the plane went down as it approached the islands. The body of the Venezuelan co-pilot later washed ashore, but no wreckage was recovered.

"These are very small planes, which have crashed into the water, an impact where they are completely lost to sight, completely destroyed," Serpi said.

"And it's very difficult, even from the vantage point of an overflight from 200 or 300 meters to survey the surface, a wide area and maintain a focused search," he said. "It's very difficult to recognize pieces."

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Ian James on Twitter: http://twitter.com/ianjamesap