Investigators searching through the wreckage of the doomed Germanwings plane in the French Alps were forced Monday to resume the hunt on foot as bad weather hampered helicopter flights.

"The teams will get to the site via the path that is already in existence," said Yves Naffrechoux of the local mountain police.

Authorities are hoping to identify more DNA strands from the 150 people who died in Tuesday's crash, as well as locate the jet's second black box that should provide more clues as to the circumstances of the tragedy.

Forensic teams have isolated almost 80 distinct DNA strands from the pulverised aircraft and have described the grim task as "unprecedented" given the tricky mountain terrain and the speed at which the plane smashed into the rock.

French officials have said that co-pilot Andreas Lubitz deliberately shut his pilot colleague out of the cockpit and then locked the Airbus A320 on a collision course with the French Alps.

READ ALSO: Germanwings: Alps crash captain shouted 'open the damn door'

The flight, en route to Duesseldorf from Barcelona, crashed into the mountainside at a speed of 700 kilometres (430 miles) an hour, instantly killing all on board.

Difficult mountain terrain

Authorities are hoping to build a more sturdy road to the crash site -- both to ease access for forensic experts and for families who want to see where their loved ones perished.

A bulldozer and several specialised machines were already working away at the site to bolster the road access.

Rescue workers are currently having to hike around three-quarters of an hour to the crash site through difficult mountain terrain, Naffrechoux said.

As on previous days, there are about 50 personnel at the crash site, he said.

"The black box has been the major target from the beginning and even more so today," he said.

Authorities say the working conditions at the inhospitable crash site have hugely slowed their progress.

Not a single body is intact, said Patrick Touron, deputy director of the police's criminal research institute.

"We have slopes of 40 to 60 degrees, falling rocks, and ground that tends to crumble," he said.

"Some things have to be done by abseiling. Since safety is key, the recovery process is a bit slow, which is a great regret."

Between 400 and 600 body parts were being examined, Touron said.

READ ALSO: DNA from 78 Germanwings crash victims found

Investigators in both France and Germany are trying to build up a picture of what could have driven Lubitz to crash his plane deliberately.

Media reports have emerged that the 27-year-old suffered from eye problems, adding to earlier reports he was severely depressed.

German prosecutors believe Lubitz hid an illness from his airline but have not specified what it was, and said he had apparently been written off sick on the day the Airbus crashed.