Here are the most crucial issues we know about so far in the Egyptair crash mystery

The EgyptAir flight MS804 connecting Paris to Cairo crashed into the sea last Thursday, most likely killing the 66 people onboard - and so far, nobody knows why.

Egypt is currently gathering flight data from airspace officials in Greece and France, as well as conducting a sea search.

Meanwhile, here are the key issues currently surrounding the crash and the search for the plane:

The battery life on the black box is running out.

French and Egyptian government officials, military people and aviation experts all say the same thing: We won't know what happened until the black box and voice recordings of the cockpit are found.

But the time to find those is slowly running out. The battery life on a black box beacon normally lasts 30 days.

The search is slightly easier because the plane went down in the Mediterranean, as opposed to a larger ocean like the Atlantic. But the plane was fitted with ultrasonic pulse technology that dates back to the 1960s, according to journalist and author Clive Irving in the Daily Beast, which stacks the chances against the search parties. The bottom of the Mediterranean has tides that could quickly cover the wreck with mud and silt.

There is only one recent case where the wreck of an airplane was found before the beacon stopped emitting pings for search ships to detect, Irving says. That was in 2006 when an Armenian airline Airbus A320 crashed into the Black Sea.

Forensic examination of recovered body parts may offer clues.

It looked like Tuesday brought some more concrete information about what might have happened as forensic experts have started to analyse the remains found by Egyptian authorities.

The size of the human remains brought to Cairo pointed to an explosion, a forensic expert claimed. "There isn't even a whole body part, like an arm or a head," a senior Egyptian forensics official told the Associated Press. "The logical explanation is that an explosion brought it down, but I cannot say what caused the blast." So far, no traces of explosives have been found.

A few hours later, though, Hisham Abdelhamid, the head of Egypt's forensics authority, said that those were "mere assumptions" and that it was too early to draw any conclusions, according to Reuters. Aviation experts have said that neither deliberate sabotage nor a technical fault could be ruled out.

The Egyptian authorities may be incompetent.

Some aviation-safety experts have also expressed concerned about the way the Egyptian authorities have been handling the debris, saying they could compromise evidence that is central to finding out what happened to the plane, the Wall Street Journal reports.

The jetliner was previously vandalised two years ago by aviation workers at Cairo Airport, who wrote in Arabic on its belly, "We will bring this plane down." That graffiti was political and appears as yet to be unrelated to the crash. Nonetheless, it does not speak well of Egyptian airport maintenance crews.

Smoke was detected in "multiple places" within the plane.

One of the most tantalising clues was the publication of data posted online by the Aviation Herald which showed that different alerts went off within 3 minutes just before the plane went off the radar.

The data shows that there were issues with the windows on the co-pilot's side of the plane. It also showed two smoke alerts, the first one "SMOKE LAVATORY SMOKE" at 00:26 a.m. local time and the second one "AVIONICS SMOKE" at 00:27 a.m. The second alert refers to the part of the plane with most of the electronics, which is located under the cockpit.

A bomb or fire could have set those alerts off, but as Bloomberg pointed out, when a bomb destroys a plane, it grills the electrical systems almost instantly.

An explosion could also have been triggered by a fire, especially if this one started under the cockpit, close to where the emergency oxygen supply for pilots is kept in the Airbus A320.

The fire may not have been the cause of the crash.

All appeared normal on the flight until the final 3 minutes when it disappeared. With smoke filling the cabin and pilots either too busy or unable to alert ground control, the end came suddenly: "Alarms went off warning about the plane's autopilot and wing control systems, suggesting serious structural problems," when the plane entered Egyptian airspace, according to the Times. Fires do not normally finish off a plane in just three minutes, according to aviation security expert Philip Baum. That suggests the damage to the plane was more catastrophic than a mere fire.

Why did the pilots not send a mayday signal?

Some basic questions remain completely unanswered. The most important one being why the pilots did not send an emergency signal or mayday for help. Egyptian airspace officials hailed the plane while it was still in the air but it did not respond. Although a terrorist attack remains one of the most probable causes, according to experts, no group has yet claimed responsibility for the attack.

The Eritrean connection.

Eritrea is Africa's North Korea - a hermit kingdom run by a brutal military dictatorship that shuns the outside world. The plane flew there the day before the crash, according to the New York Times. The US State Department warned in May 2015 that security at the Asmara airport "can be unpredictable," and had a "lack of efficiency and consistency" in screening passengers, the NYT said.

The plane once had a faulty engine.

The Airbus A320's engine overheated once in 2013, but experts do not believe this problem would be severe enough to bring down the plane on its own.

Did the plane swerve before it disappeared?

Immediately after the disappearance, the Greek defence ministry said the plane had swerved 90 degrees, then 160 degrees, and went down from 37,000 feet to 11,000 feet just before going off the radars. Many people suggested that was evidence of a possible hijack of the plane, with the pilots struggling over the controls.

But on Monday, senior Egyptian officials said that the plane did not swerve or lose altitude, completely refuting initial Greek reports. That official also said that the plane entered the Egyptian airspace and was tracked by Egyptians for a minute or two before going off the radars, the Guardian reports.