Montabaur, Germany (CNN) Co-pilot Andreas Lubitz's voice isn't heard in a recording of the final moments of Germanwings Flight 9525.

But investigators say he was the one at the controls inside the cockpit, deliberately locking out the plane's captain and setting the plane on a crash course for the French Alps.

The only sound the recorder picked up from Lubitz as the Airbus A320 went down, Marseille prosecutor Brice Robin said, was the co-pilot's steady breathing.

Authorities didn't mince words Thursday as they blamed Lubitz for the crash , but questions are still swirling over the 27-year-old's past.

Officials say Lubitz passed a psychological test when he was hired, had no known ties to terrorism and showed no sign of medical distress during the flight.

It seems, Robin said, that Lubitz "wanted to destroy the aircraft."

But why?

Police search apartment

It's a question police were trying to answer as they searched Lubitz's apartment in Dusseldorf.

Police spokesman Markus Niesczery said a team of five investigators went "through the apartment looking for clues as to what the co-pilot's motivation might have been, if he did indeed bring the plane down."

About 85 miles (136 km) away in the town of Montabaur, the house where Lubitz's parents live was shuttered and guarded by police.

A group of men, perhaps investigators, were the only ones granted access.

'This is just inconceivable'

This town in western Germany is where Lubitz pursued his love of flying from a young age.

At a club on the outskirts of Montabaur, pilots who knew Lubitz said they were shocked to hear what investigators said.

They said the man they know never would have deliberately crashed a plane.

Between the ages of 14 and 20, Lubitz was a regular fixture at the gliding club.

"(He was) a very normal young person, full of energy," Klaus Radke said. "What can I say? He had a bright future. He made his hobby into his job. What more can you hope to achieve?"

The authorities' explanation doesn't ring true for Peter Ruecker, another pilot who knew him from the flight club.

"Knowing Andreas, this is just inconceivable for me," Ruecker told the Reuters news agency.

"He was a lot of fun, even though he was perhaps sometimes a bit quiet," Ruecker said. "He was just another boy, like so many others here."

A neighbor told Reuters that Lubitz "was very interested in things which are going on around him."

"It's a very good family," the neighbor said. "They have a good connection within the family and they are engaged in the community."

'Interrupted' training

Lubitz had been with Germanwings, a budget airline owned by Lufthansa, since September 2013 and had completed 630 hours of flight time, the airline's media office said.

Lufthansa CEO Carsten Spohr told reporters that Lubitz "interrupted" his training, which he began in 2008. That break lasted several months, he said, but added that such an interruption isn't uncommon.

Spohr said he couldn't give any information about why the co-pilot had stopped and then restarted his training.

If it was for medical reasons, he said, then that information would have been private before the crash, he said, but it will be part of information gathered during the investigation.

Most of Lubitz's training took place at the Lufthansa flight training center in Bremen.

He also trained in the United States, spending six months at facility in Arizona as part of a required program to get his license, a Lufthansa spokesperson said.

Spohr said Lufthansa pilots get medical testing but do not undergo regular or routine psychological testing once they are flying. However, the airline does consider an applicant's psychological state, along with other factors, when hiring pilots, he said.

Lubitz and the captain passed a psychological test when they were hired, he said.

"We don't only look at competence but we also give a lot of room to psychological capabilities," Spohr said.

"He was 100% set to fly without restrictions," he added. "His flight performance was perfect. There was nothing to worry about."