Air traffic controllers in Switzerland say an automatic warning system was switched off and only one staff member was on duty when a Russian airliner and a cargo jet collided.

Skyguide - the Swiss air traffic control company responsible for planes at the time of the crash - said the system, which should have warned ground control that the two planes were on a collision course, was out of action for routine maintenance.

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The search for the remains of the 71 people killed is continuing at the crash site near Lake Konstanz on the Swiss-German border.

So far, 37 bodies have been recovered, along with aircraft parts which are being taken to a hangar to be reconstructed.

Ground control under pressure

A spokesman for Skyguide told German television that it remained a "hypothetical question" whether the warning system or the presence of the second controller - who had gone on a break - could have prevented the mid-air crash.

There has been confusion about whether or not the second controller broke the rules when he agreed with his colleague to take a break.

	:	Mid-air collision
How the crash happened

Skyguide initially said that under its regulations, two members of staff should be on duty when the early warning system is off line - but later said if skies were quiet breaks were allowed.

The BBC's Imogen Foulkes, in Switzerland, says confusion caused by several conflicting statements from Skyguide is turning into a public relations disaster for the company.

An investigation by Swiss authorities into two previous near misses has also raised questions about Skyguide's accuracy and quality.

In a report published just last week, they pointed to insufficiencies in its radars and serious problems in its co-ordination with neighbouring countries and military air controllers, Spiegel Online reported.

A German prosecutor has opened a case of death by negligence and plans to question Skyguide, Swissinfo radio website reported.

Controller in shock

The controller who was left alone to deal with the situation is currently being treated for shock and has so far been unable to give his version of events.

	Tupolev 154 flying from Moscow to Barcelona Boeing 757 flying from Bergamo to Brussels Collision happened on 1 July at 2130 GMT. All lives aboard lost What went wrong?

Investigators say he issued a warning to the Russian pilot to lower his altitude 50 seconds before the two planes collided. The pilot only began descending on a second warning, issued just 25 seconds before the crash.

By then, a safety warning device on board the cargo jet meant the pilots too were plunging downwards and - instead of avoiding each other - the two planes collided, bursting into a fireball.

Skyguide says the 50-second warning was sufficient to prevent the collision, but Russian officials say it came too late.

Search for remains

German and Russian investigators - along with US colleagues - have been inspecting the scene and have begun analysing data retrieved from the flight data and cockpit voice recorders of the two planes involved.

Unanswered questions

Why were the Russian pilots not informed earlier of the collision course? Why did they not react sooner? Why did the automatic anti-collision systems on board both planes failed to interact correctly?

Hundreds of police officers and recovery workers have been scouring the hillsides above Ueberlingen, collecting any debris that may offer clues.

"It's a very difficult recovery operation," said police officer Harold Hanner. "The wreckage is spread over 30 kilometres, much of it in fields and rural areas difficult to access".

The BBC's Emma Jane Kirby at the crash site says there are fragments of charred seat covers, pieces of what looked like burnt clothing and little bits of metal everywhere.

Only the bodies of the Boeing's two pilots have so far been identified.

In one part of the Russian Tupolev's fuselage, the victims are reported still to be strapped into their seats.

Relatives of the 52 children and teenagers who died on board the Tupolev are due to fly in from Russia to view the scene. A number of adults were also on board the airliner.