As the search for a missing Malaysia Airlines jet entered a fourth day Tuesday, investigators remained uncertain about its whereabouts.
Here's a summary of what we know and what we don't know about Flight 370, which was carrying 239 people when it disappeared from radar screens over Southeast Asia.
THE FLIGHT PATH
What we know: The Boeing 777-200ER took off from Kuala Lumpur, the Malaysian capital, at 12:41 a.m. Saturday (12:41 p.m. Friday ET). It was scheduled to arrive in Beijing at 6:30 a.m. the same day, after a roughly 2,700-mile (4,350-kilometer) journey. But around 1:30 a.m., air traffic controllers in Subang, outside Kuala Lumpur, lost contact with the plane over the sea between Malaysia and Vietnam.
What we don't know: What happened next. The pilots did not indicate any problem to the tower, and no distress signal was issued. Malaysian military officials cite radar data as suggesting the plane might have turned back toward Kuala Lumpur. But the pilots didn't tell air traffic control that they were doing so. And we don't know why the plane would have turned around.
What we know: There were 239 people on board: 227 passengers and 12 crew members. Five of the passengers were younger than 5 years old. Those on board included a number of painters and calligraphers, as well as employees of an American semiconductor company.
According to the airline, the passengers' 14 nationalities spanned the Asia-Pacific region, Europe and North America. Passengers from China or Taiwan numbered 154, followed by Malaysians, at 38. There were three U.S. citizens on the plane. Four passengers had valid booking to travel but did not show up to for the flight, according to the airline. "As such, the issue of off-loading unaccompanied baggage did not arise," it added Tuesday in a prepared statement.
What we don't know: Why two people who boarded the plane were using stolen passports, officials say.
THE PASSPORT MYSTERY
What we know: The tickets for the two people who used stolen Italian and Austrian passports were bought Thursday in Thailand, according to ticketing records. Both tickets were one-way and had itineraries continuing on from Beijing to Amsterdam. One ticket's final destination was Frankfurt, Germany; the other's was Copenhagen, Denmark. The passports were stolen in Thailand from the two people to whom they had been issued -- the Austrian's was taken last year and the Italian's in 2012.
Interpol identified the men using the stolen passports as Pouri Nourmohammadi, 18, and Delavar Seyed Mohammad Reza, 29, both Iranians. Malaysian police believe Nourmohammadi was trying to emigrate to Germany using the stolen Austrian passport. The men entered Malaysia on February 28 using valid Iranian passports.
What we don't know: Whether the stolen passports have any connection to the plane's disappearance.
Would-be immigrants have used fake passports to enter Western countries in the past.
THE SECURITY SCREENING
What we know: Interpol says the passports were listed as stolen in its database. But they had not been checked from the time they were entered into the database and the time the plane departed. Interpol Secretary General Ronald K. Noble said it was "clearly of great concern" that passengers had been able to board an international flight using passports listed as stolen in the agency's database.
What we don't know: Whether the passports had been used to travel previously. Interpol says it's "unable to determine on how many other occasions these passports were used to board flights or cross borders." Malaysian authorities are investigating the security process at the airport in Kuala Lumpur, but insisted it meets international standards.
What we know: The crew members are Malaysian. The pilot is Capt. Zaharie Ahmad Shah, a 53-year-old veteran with 18,365 flying hours who joined Malaysia Airlines in 1981. The first officer, Fariq Ab Hamid, has 2,763 flying hours. Hamid, 27, started at the airline in 2007. He had been flying another jet and was transitioning to the Boeing 777-200 after having completed training in a flight simulator.
What we don't know: What went on in the cockpit around the time the plane lost contact with air traffic controllers. The passenger jet was in what is considered the safest part of a flight, the cruise portion, when it disappeared. The weather conditions were reported to be good. Aviation experts say it's particularly puzzling that the pilots didn't report any kind of problems before contact was lost.
What we know: Thirty-four planes, 40 ships and search crews from 10 countries are scouring the South China Sea near where the plane was last detected. Debris in the area has turned out to be unrelated to the plane. "We have not found anything that
appear to be objects from the aircraft, let alone the aircraft," Azharuddin Abdul Rahman, director general of the Malaysian Civil Aviation Department, said Monday. Similarly, a slick in the area was determined to be from fuel oil typically used in cargo ships, not from the plane.
What we don't know: Whether the search is concentrating on the right place. Authorities initially focused their efforts around the mouth of the Gulf of Thailand, near the plane's last known position. But they have expanded efforts westward, off the other coast of the Malay Peninsula, and northward into the Andaman Sea, part of the Indian Ocean.
What we know: Nothing. "For the aircraft to go missing just like that ... as far as we are concerned, we are equally puzzled as well," Rahman said Monday. The aircraft model in question, the Boeing 777-200ER, has an excellent safety record.
What we don't know: Until searchers find the plane and its voice and data recorders, it may be difficult to figure out what happened. CNN national security analyst Peter Bergen says the range of possible reasons behind the disappearance can be divided into three categories: mechanical failure, pilot actions and terrorism. But all we have are theories.
What we know: It's rare, but not unprecedented, for a commercial airliner to disappear in midflight. In June 2009, Air France Flight 447 was en route from Rio de Janeiro to Paris when communications ended suddenly from the Airbus A330, another state-of-the-art aircraft, with 228 people on board. It took four searches over nearly two years to find the bulk of Flight 447's wreckage and most of the bodies in a mountain range deep in the Atlantic Ocean. It took even longer to establish the cause of the disaster.
What we don't know: Whether what happened to the missing Malaysia Airlines plane is similar to what happened to the Air France flight. Investigators attributed the Flight 447 crash to a series of errors by the pilots and their failure to react effectively to technical problems.