"We at Lufthansa are speechless that this aircraft has been deliberately crashed by the co-pilot," said Carsten Spohr, CEO of Lufthansa, which owns Germanwings.

FBI assisting in crash investigation

Marseille prosecutor Brice Robin said the co-pilot, 28 year old German National Andreas Lubitz,apparently "wanted to destroy the aircraft."

It's unknown whether Lubitz planned his actions, Robin said. But he "took advantage" of a moment in which the pilot left the cockpit and "activated the descent," which can only be done deliberately.

Andreas Lubitz

It's also unclear whether the pilot entered a code to try to get back into the cockpit, or whether Lubitz "put the lever on lock," which would have prevented the code from working, Spohr said.

What is known is that screaming could be heard on an audio recording for the final few minutes of the flight.

Death was instantaneous for those on board when the plane plunged into the mountains, Robin said.

German authorities also say "that we have to assume" that Flight 9525 "was deliberately activated for a crash," German Chancellor Angela Merkel said Thursday. "This is hard to comprehend for most people. Nobody can imagine this."

The French government has asked the FBI to help investigate the crash, a law enforcement official said Thursday.

A search is underway for the plane's second "black box," the flight data recorder, which could shed more light on the plane's final minutes.

No clues about why co-pilot would crash plane

Investigators so far say they're baffled about why Lubitz would have done this

Lufthansa does "not have any clues," Spohr said.

The picture of the plane's final minutes comes largely from what was discovered in the mangled cockpit voice recorder.

Lufthansa CEO

The pilot and co-pilot had normal exchanges during the beginning of the flight, Robin said. When the pilot stepped out to go to the bathroom, he asked Lubitz to take over.

The most plausible explanation of what happened next is that Lubitz, "through deliberate abstention, refused to open the cabin door ... to the chief pilot, and used the button" to cause the plane to lose altitude, Robin said.

The disaster is not being described as a "terrorist attack," and the killing of 150 people would generally not be described as a "suicide" either, Robin said. Spohr agreed: "If a person kills himself and also 149 other people, another word should be used -- not suicide," he said.

Lubitz was not known to be on any terrorism list, and his religion was not immediately known, Robin said.

Lufthansa has no standard psychological testing after hiring

He had been with Germanwings since September 2013 and had completed 630 hours of flight time, the company said. Lubitz had trained at the Lufthansa flight center in Bremen, Germany.

He only had about 100 hours of experience on the type of aircraft he was flying, but he had all the necessary certifications and qualifications to pilot the aircraft alone, the prosecutor said.

He had passed medical tests,Sophr said.The audio recording showed his breathing to be steady, with no sign that he had a heart attack or other medical issue.

Lufthansa does not have standard psychological testing for pilots once they are hired, Spohr said. The company considers an applicant's psychological state when hiring, he said.

The co-pilot was "fully qualified to pilot the aircraft on his own," Robin said.

A man in Montabaur, Germany, who belonged to the same flight club as Lubitz, said he couldn't believe it. "The way I know Andreas, this is inconceivable," Peter Ruecker said Thursday.

Village opens homes to victims' loved ones

Relatives and friends of the victims traveled Thursday on special Lufthansa flights to an area near the site where their loved ones perished.

Seyne-les-Alpes, a nearby town, is serving as a staging post. Mayor Francis Hermitte predicted that 200 to 300 people would come to the area Thursday.

Most are not expected to stay overnight, he said. But in case they do, he said, local residents have offered accommodations for them.

Lufthansa is providing "financial support" to relatives of the victims, Spohr said. He declined to go into details.

The families of the two pilots are also in France, Robin said, but they are not in the same place as the passengers' relatives.

The bodies of the crash victims will not be released to family members until all DNA identification work has been done -- a process likely to last several weeks, he said.

While some human remains have been recovered, many have not. The task is treacherous for search crews working on steep slopes in icy weather. Workers were dropped by helicopters and tied together for safety