Debris found in the Mediterranean Sea "does not come from a plane," Athanasios Binis, the head of the Greek air safety authority, told Agence France-Presse.
He told the French news agency that plane wreckage has not been found. The AFP account matches an EgyptAir retraction to CNN that the wreckage of Flight 804 had been found.
Earlier, EgyptAir Vice Chairman Ahmed Adel told CNN's Christiane Amanpour that "we have found the wreckage." The airline said that life jackets and plastic materials believed from the flight had been found near the Greek island of Karparthos. The airline now says that is not correct.
The Airbus A320 carrying 56 passengers and 10 crew and security officers disappeared early Thursday over the Mediterranean as it flew from Paris to Cairo.
Speculation has centered on the possibility of a terrorist attack.
"It's very difficult to come up with a scenario that jibes with some sort of catastrophic failure. (The evidence so far) leads us down the road to a deliberate act.," CNN aviation analyst Miles O'Brien said.
Egyptian Civil Aviation Minister Sharif Fathi said technical failures and terror are each possible explanations.
"But if you analyze this situation properly, the possibility of having a different action aboard, of having a terror attack, is higher than having a technical problem," Fathi said.
-- The debris found includes life jackets and plastic materials, the airline said.
-- Maintenance checks on the plane had been done on time and "no snags were reported," Adel told Amanpour.
-- Checks of the passenger manifest have so far resulted in no hits on terror watch lists, officials with knowledge of the investigation told CNN.
-- U.S. government officials are operating on an initial theory that the plane was taken down by a bomb, two U.S. officials told CNN. Officials said the theory could change, with one senior administration official cautioning it is not yet supported by a "smoking gun."
-- President Barack Obama has been briefed on the situation, said Lisa Monaco, assistant to the president for homeland security and counterterrorism.
-- The airplane "swerved and then plunged" before descending into the Mediterranean, Greek Defense Minister Panos Kammenos told reporters.
-- Greek controllers tried to reach EgyptAir Flight 804 about 10 miles before it left the country's airspace and for about 90 seconds after and received no response, the head of the Hellenic Civil Aviation Authority told Greek broadcaster ANT1 TV.
The flight seemed to be proceeding normally until it approached Egyptian airspace. Greek controllers talked to the pilot when the plane was near the Greek island of Kea at 37,000 feet at an air speed of 519 mph. Everything seemed fine at that point.
At 2:27 a.m., shortly before the aircraft was scheduled to exit Greek airspace, controllers tried to reach the pilots to transfer control to Cairo authorities. Despite repeated attempts, they received no response, the Hellenic Civil Aviation Authority said. The plane passed into Egyptian airspace two minutes later. Forty seconds later, radar contact was lost, the authority said.
Weather conditions were clear at the time, CNN meteorologist Pedram Javaheri said.
At 2:29 a.m., just after it had entered Egyptian airspace, the plane swerved 90 degrees to the left, and then 360 degrees to the right before plunging first to 15,000 feet, then 10,000 feet, before dropping off radar, Kammenos, the Greek defense minister, told reporters.
More about the flight
The flight left Charles de Gaulle Airport in Paris at 11:09 p.m. Wednesday for what should have been about a 3½-hour flight.
Aboard were 56 passengers and 10 cabin crew members and security officers. The passengers were predominantly Egyptian -- 30 in all -- but also aboard were 15 French citizens, including an infant; two Iraqis; and one from each of the following countries: Britain, Belgium, Kuwait, Saudi Arabia, Sudan, Chad, Portugal, Algeria and Canada, according to Fathi, the Egyptian aviation minister.
The government of Canada said two of its citizens were on the plane. The reason for the discrepancy wasn't clear but there is possibility one or more had dual citizenship.
The Airbus A320 had routine maintenance checks in Cairo before it left for Paris, an airline official said. Earlier Wednesday, the jet was also in Eritrea and Tunisia, data from flight tracking websites show.
There was no special cargo on the flight and no notification of any dangerous goods aboard, according to Adel of EgyptAir.
The plane has been part of EgyptAir's fleet since November 2003, according to Adel. It had about 48,000 flight hours. The plane's captain had about 6,000 flying hours, he said.
A distress signal was detected at 4:26 a.m. -- about two hours after the jet vanished -- in the general vicinity where it disappeared, Adel said.
He said the distress signal could have come from another vessel in the Mediterranean. Egyptian armed forces said they had not received a distress call.
The area being searched is about 130 nautical miles south-southeast of Karpathos, the Greek Hellenic National Defense General Staff said.
The Greek military was sending two aircraft, two helicopters and a frigate to the crash site. The U.S. Navy had deployed a P-3 Orion aircraft to assist in the search. Egypt's military was also involved, and France said it had tasked a surveillance plane to help.
A storm system could affect conditions in the region as early as Friday afternoon, Javaheri said.
As crews searched, somber relatives gathered in Cairo and Paris airports, seeking word on their loved ones.
They were taken to special centers at both airports, where translators and psychiatric support awaited. In Cairo's airport, dozens of relatives paced anxiously in a building set aside for families. Some shouted at photographers taking pictures of them, while others berated officials over the perceived lack of information.
Analysts weigh in
CNN aviation correspondent Richard Quest: "Planes just do not fall out of the sky for no reason, particularly at 37,000 feet," he said, noting the aircraft vanished while cruising -- the safest part of the journey.
David Soucie, a CNN aviation safety analyst: The first priority is to find survivors. "Find the plane, find the people, see if there are folks that could be rescued," he said. "Safety people are looking at safety issues, maintenance people looking at maintenance issues, security people looking at security issues."
CNN aviation analyst Les Abend: He said there are three possibilities: an explosion, something nefarious or a stall situation. "We're in the very early stages of the investigation. Any good accident investigator will tell you, just put on the brakes a little bit and let this thing unfold. The 360-degree turn, that seems very abrupt. It's not something I would do in any major emergency unless I was losing control of the aircraft," he said.
Egypt's aviation incidents
Egypt is no stranger to aviation disasters.
In March, an "unstable" man diverted an EgyptAir flight from Alexandria to Cyprus. The suspected hijacker later released all hostages and surrendered.
Last year, a Russian plane exploded midair over the Sinai Peninsula, killing all 224 people aboard. Egyptian officials initially downplayed Islamic militants' claim that they brought down the jet, saying technical failure caused the crash.
And in October 1999, an EgyptAir passenger jet made a rapid descent, plunging almost 14,000 feet in 36 seconds.
The Boeing 767, en route to Cairo from New York, crashed into the Atlantic Ocean off the Massachusetts coast.
Its debris was later found, but speculation remains on the cause of the crash that killed all 217 people on board.