Investigators have been busy reconstructing the events that led to Germanwings Flight 9525 crashing in the French Alps on Tuesday. A day later, there were still many critical questions left unanswered.

"We cannot comprehend how a technically flawless airplane steered by two experienced pilots could encounter such a situation,” a spokesperson for Lufthansa, which owns Germanwings, tweeted Wednesday. “We cannot believe that this has happened. We are doing everything to support the families.” All 150 passengers and crew members were killed in the puzzling plane crash.

In the past 24 hours since the disaster, officials have been able to deduce that the aircraft was traveling at roughly 300 kilometers per hour (480 miles per hour) before losing contact with air traffic controllers, that its last known altitude was about 3,470 meters (11,400 feet), and that at no time did the pilot send out a distress signal. Germanwings Flight 9525 left Barcelona, Spain, at 10:01 a.m. local time Tuesday. It was scheduled to land in Düsseldorf, Germany, about an hour and a half later.

Dozens of helicopters and hundreds of police and rescue workers descended on the French Alps region near Digne-les-Bains, France, where the aircraft went down. Aerial views of the crash site, located near a popular ski resort and set back from mountain roads, showed a mountainside littered with debris and bodies.

"Seeing the site of the accident was harrowing," Lufthansa CEO Carsten Spohr said Wednesday via Twitter. “We are in deep mourning. Our thoughts are with the relatives of the victims.” Authorities stood guard at the site overnight to keep journalists, souvenir gatherers and wild wolves from disrupting the crash zone.

Much of what remains unexplained about the accident rests with the aircraft’s black-box recorders, a set of instruments that measure everything from airspeed and altitude change to the direction the plane was headed at every moment. The aircraft’s cockpit voice recorder was meant to capture conversation between the plane’s pilots.

The instruments were badly damaged in the crash, but useable, and will be analyzed for information, investigators said. Here are five things officials don’t know about the Germanwings Flight 9525 crash.

1. If the plane’s autopilot was engaged at the time of the crash. Determining whether the plane’s pilots, who have not been identified, were in control of the aircraft when the plane made a sudden and unexpected descent from its cruising altitude is a critical piece of missing information.

2. What occurred in the two minutes around 10:30 a.m. local time. That’s when the pilots stopped responding to ground control, according to the Independent. Exactly why the plane suddenly went silent could prove vital to understanding the reason for the crash.

3. Why the plane inexplicably descended after reaching its cruising altitude. The plane’s 18-minute descent toward the mountains could suggest engine failure; however, that can’t be determined until investigators can retrieve the black-box data.

4. Whether the pilots were alone in the cockpit. While officials have ostensibly ruled out foul play, listening to the plane’s cockpit voice recorder will allow them to know for sure whether a third person was in the room before the crash.

5. Was there a technical failure? Despite the Airbus A320’s reputation as one of the safest planes ever made, some have raised suspicions that the plane wasn’t fit for flying. The aircraft had been grounded for an hour before its Tuesday departure because of a problem with its nose wheel door, according to Metro. However, officials said the issue had been resolved prior to departure.