Germanwings crash investigators turn to FBI for help

US law enforcement analyst Tom Fuentes says the FBI will help zero in on what was behind the crash.

Rescuers prepare in a field where the rescue effort is headquartered on 24 March, 2015 in the southeastern French town of Seyne after a German Airbus A320 of the low-cost carrier Germanwings crashed, Picture: AFP.

JOHANNESBURG - The Federal Bureau of Investigation (FBI) has confirmed it's been formally asked to assist with the investigation into the disaster.

It emerged this afternoon that Andreas Lubitz refused to open the cabin door and deliberately caused the plane's loss of altitude after ignoring messages from air traffic control.

Lubitz also ignored the captain's frantic pounding on the locked cockpit door, as well as the screams of passengers.

US law enforcement analyst Tom Fuentes says the FBI will help zero in on whether it's a crime, an act of terror or human error.

"The security measures worked too well if it... (kept) the bad guy in & the good guys out." Tom Fuentes #Germanwings

"The host country immediately asked the FBI to start looking at the passenger manifest, the crew, anybody who came close the airplane when it was on the ground, if they think there was sabotage involved."

The plane went down in a remote region of the French Alps on Tuesday morning killing all 150 people on board. Two babies were among the passengers.

WATCH:_ A simulation of what happened._

Families of victims on board the plane have started arriving at the crash site for a memorial service.

CNN's Nic Robertson says this is the closes location that the families will be brought to.

"There is a memorial plaque in the field behind me, there are emergency services there, medical professionals on hand to help with psychological issues with the families, they will see bouquets of flowers, flags have been put out to represent the different victims aboard the flight."

Marseille prosecutor Brice Robin described this afternoon how families of the victims reacted with shock and posed countless questions after learning of the screams of their loved ones just moments before the plane went down.

"The families were in shock, because they just heard what happened, they found it difficult to believe. I tried to give them answers, it took at least an hour and a quarter. I tried to answer as best I could."

Meanwhile, Lufthansa CEO Carstens Sphor says he has been left speechless but will help all the families.

"We will give financial support to relatives and that was said in Barcelona and Dusseldorf so that we facilitate this as quickly as possible so that we can give support."

Sphor says the revelation that Lubitz deliberately crashed the Airbus was the company's worst nightmare.

Lufthansa says its pilots undergo annual medical checks but not psychological assessments beyond training.

The carrier also says it has faith in its company's hiring and vetting procedures but will now review the process with experts.

LISTEN: Air travel still safer than road travel.