More than 24 hours after an EgyptAir jet disappeared over the Mediterranean Sea with 66 people aboard, the cause of the apparent crash and location of the plane was still unknown Thursday.
The Airbus A320 traveling from Paris to Cairo had exited Greek airspace and entered Egyptian airspace when it vanished from radar at around 2:30 a.m. Thursday Cairo time (8:30 p.m. Wednesday ET), the airline and Greek officials said.
The search for the plane focused on the Mediterranean Sea. The plane lost contact with radar about 170 miles from the Egyptian coast, the airline said. The Egyptian air force and navy as well as Greek aircraft and ships were involved in the search.
As family members wait for word, many details remain unclear. The airline announced that wreckage from the plane had been found near the Greek island of Karpathos, but hours later retracted the claim, and said the debris was not from the aircraft.
Officials don't know for certain. Egyptian officials suggested Thursday that terrorism was more likely the cause than mechanical issues.
While still nearly 200 miles away from landing, the plane, traveling at 37,000 feet, "executed a turn of 90 degrees left and then a turn of 360 degrees toward the right, dropping from 37,000 to 15,000 feet, and then the picture we had was lost at about 10,000 feet," Greek Defense Minister Panos Kammenos said Thursday.
And a senior U.S. intelligence official familiar with the region told NBC News infrared and multi-spectral images strongly suggest there was an explosion aboard the plane.
"It's not conclusive, but it's suggestive," a U.S. administration official concurred. ""Now, the question is, if there was an explosion, what caused it? Mechanical failure? Explosives? No idea at this point."
The altitude of the plane when it left radar screens suggests "some sort of abrupt incident, as opposed to some sort of gradual malfunction" said Daniel Nisman, a security analyst at the Levantine Group.
"It doesn't exhibit the normal features of something accidental," he said.
Airbus said the missing Airbus A320 was made in 2003 and delivered to EgyptAir in 2008, adding that the aircraft had accumulated around 48,000 flight hours. The weather was reported clear at the time of the disappearance.
Has any terror group claimed responsibility?
There have been no credible claims. FBI Director James Comey said Thursday, "so far at least we have no claim of responsibility or evidence that this was an intentional act."
American intelligence officials were running the names on the passenger manifest through various terrorism databases, but nothing has emerged indicating terrorism, a U.S. official told NBC News.
There has been no comment from media affiliated with the terror group ISIS, according Flashpoint Intelligence, a global security firm and NBC News consultant.
ISIS has claimed responsibility for the downing of a Russian Metrojet plane which crashed in Egypt's Sinai Peninsula on Oct. 31, killing all 224 people on board. Investigations have not established the veracity of the claim.
And in February, a Somali plane was forced to land after taking off from Mogadishu after an explosion that officials suspect was caused by a bomb. The only person killed was the bomber who was blown out of the plane. Islamic extremist group al Shabab claimed responsibility.
An EgyptAir flight was hijacked in March and forced to land in Cyprus, but officials described the hijacker as emotionally disturbed and motivated over problems with an ex-wife. The claimed explosive belt he was carrying turned out to be a fake. No one was hurt.
What has been found?
Debris was spotted in the sea, prompting the airline to announce that wreckage of the plane had been found, but it was later determined none of the materials came from the Egyptair plane.
EgyptAir, citing a government letter sent to Egypt's Ministry of Civil Aviation, said "wreckage of the missing aircraft No. MS 804" was found near Karpathos Island, without elaborating on what exactly was discovered.
Hours later, the airline retracted that statement and said it received incorrect information.
"We stand corrected on that," Ahmed Adel, Vice President of EgyptAir, told CNN. Adel said he had initially received information "through some official channels" that the wreckage had been found.
"Then later on after they got close to what was thought was the wreckage of EgyptAir, they realized it's not our aircraft," he said.
Athanassios Binis, head of Greece's Air Accident Investigation and Aviation Safety Board, earlier told state broadcaster ERT TV that debris that was found did not belong to an aircraft.
Who are the pilots?
The pilot was identified by EgyptAir as Mohamed Said Aly Aly Shakeer, and the airline said he had 6,275 flying hours in all, including 2,101 hours flying an Airbus A320, the type of plane that disappeared.
The co-pilot, Mohamed Ahmed Mamdouh Ahmed Assem had 2,766 flying hours, the airline said.
An Egyptian Interior Ministry official told the New York Times that both men had no known political affiliations and had passed security background checks.
The air hostess crew was identified as Mervat Zaki Zakri Mohamed, Atef Lutfy Abdel Lateef Amin, Samir Ezzedin Safwat Youssef, Haitham Mostafa Azz al Hameed Al Azzizi and Yara Hani Farag Tawfiq.
Three people aboard were security agents, the airline said. They were identified as Mohamed Ahmed Abd al Razak Abd al Kareem, Ahmed Mohamed Magdy Ahmad and Mohamed Abdel Monim al Ghoneimy al Kyal.
The pilot was cheerful with air-traffic controllers, thanking them in Greek as the plane was preparing to depart Greek airspace, the Greek Civil Aviation Authority said.
When Greek air traffic controllers repeatedly tried to contact the pilot to transfer communication to Cairo air traffic controllers, they received no response from the plane.
Who were the passengers?
Identifies have not been released. Of the 56 passengers, 30 were Egyptian, the airline said.
Fifteen passengers were French, two were Iraqis, and one each was a citizen of the U.K., Belgium, Kuwait, Saudi Arabia, Sudan, Chad, Portugal, Algeria, and Canada, according to the airline. But Canadian Minister of Foreign Affairs Stéphane Dion said two Canadian citizens were among the passengers on the flight.
Australian Foreign Affairs Minister Julie Bishop said a dual Australian-U.K. national was among those on board, but would not provide more details out of respect for the man's family. "My thoughts are with the families and friends of those affected," Bishop said.
The Kuwait News Agency said Kuwaiti citizen Abdulmohsen Al-Mutairi was on board the plane, citing the country's assistant foreign minister for consular affairs.
Ahmed Helal, director of a Procter & Gamble manufacturing site in Amiens, France, was also on the plane, the company said.
"This is a very difficult moment for P&G and in particular for our employees in Amiens," the company said in a statement. "Our priority today is to fully support Mr Helal's family during this very difficult time and all P&G employees who are very much affected by this tragedy."
No Americans were listed as being on board.
Greek and Egyptian authorities are searching for the plane in the Mediterranean. A U.S. Navy P-3 maritime surveillance plane also joined the search at the request of the Greek government.
U.S. Secretary of State John Kerry called Egyptian President Abdel Fattah El Sisi and "expressed his country's readiness to fully provide all forms of necessary assistance to Egypt to unravel the circumstances surrounding the incident," the Egyptian government said in a statement.
"The United States stands ready to provide our full support and resources to the governments of Egypt and France as they investigate this incident," White House press secretary Josh Earnest said in a statement.
France's aviation authority, the BEA, said it was in contact with Egyptian authorities and was taking part in the investigation.
The British Air Accidents Investigation Branch, meanwhile, said it had offered to assist Egypt's authorities, as well.
While terrorism has not been established as a cause, Los Angeles International Airport said in a statement Thursday that in light of the disappearance of the EgyptAir flight it has "heightened our security posture and enhanced our counter-terrorism security measures."
Comey, the FBI director, said it wasn't clear whether the agency would send anyone to help in the investigation until it becomes more clear what exactly happened to the aircraft. The FBI is working with Egypt and will share any useful information, he said.
"When something like this happens, we talk to each other, we share information and that process is going on right now," Comey said.
A former senior intelligence official told NBC News that a network of U.S. satellites called the Space Based Infrared System (SBIRS), which is designed to automatically detect missile launches worldwide, could have detected any explosion on the plane
"We have a worldwide network that detects missile launches," the former official said. "What is a missile launch? It's an explosion. If the explosion was long enough and intense enough, the system would see it."
"This is a very, very advanced aircraft; it doesn't just disappear," another former U.S. intelligence official told NBC News Thursday. "There could be many different scenarios. But for a plane to just disappear at 37,000 feet, I'm going with the worst-case scenario."