UEBERLINGEN, Germany -- Recovery work in southern Germany after a mid-air plane collision Monday was expected to continue through the weekend as forensic experts worked to identify the 69 bodies recovered so far.

The remains of two victims remained missing as of Friday night, and recovery workers were also combing the crash site and the surrounding area on lake constance for wreckage as investigators tried to piece together the cause of the accident, in which a Russian Tupolev 154 passenger jet and a Boeing 727 cargo plane collided.

Evidence so far has pointed to air-traffic control in Zurich, where a key computer and a phone line was down at the time for maintenance, and one of the two controllers had taken a break against regulations, told DPA.

Specialists have so far identified 10 of the crash victims while recovery workers at the scene encountered some difficulties. A police spokesman in nearby Friedrichshafen said they were having a hard time removing a large piece of wreckage belonging to the Russian plane from a field, where victims' relatives left behind flowers Thursday.

investigators want the wreckage to remain in one piece.

On Saturday, the city of Ueberlingen planned to erect a wooden cross in memory of the victims, and an outdoor memorial service is planned later.

But in the nearby community of owingen, a mass in memory of the victims is to be held Saturday, ending with a silent march to the crash site, Mayor Guenther Former said.

German investigators said Friday that the lone air-traffic controller at the Zurich Tower might have been distracted by the dead phone line as the two planes raced toward one another.

The skyguide air controller was trying unsuccessfully to phone another tower about landing procedures for a small plane he was watching as the two big aircraft closed in on each other, his colleague was on break and a computer was offline, investigators said.

He spent several vital minutes trying to get through to the tower in Friedrichshafen, said officials in Braunschweig, Germany, investigating the mid-air collision.

In the minutes before the crash, the lone controller was tracking five planes on two radar screens. One screen showed the small plane making its landing approach, and the other tracked four aircraft, including the two planes drawing ever closer together.

Revelation of the phone outage came as authorities in both Germany and Switzerland considered whether to file charges of manslaughter amid mounting evidence of irregularities in procedures at the skyguide air-control tower in Zurich.

Swiss investigators said they had sufficient evidence for a criminal investigation that could end in charges of negligent homicide as well as negligent endangering of public transport.

Prosecutors across the border in Germany were also looking into the case.

The criminal investigations were launched after German air-crash investigators said Swiss Air control had radioed instructions for evasive action far later than the minimum 90 seconds necessary for planes travelling at such speeds to avoid a collision.

Director of the Federal Bureau of Aircraft Accidents Investigation, Peter Schlegal, said investigators had established the Tupolev was given 44 seconds of warning by skyguide air traffic control in Zurich that it was on a collision course.

Previous accounts have suggested there was 50 seconds of warning.

Schlegel said calculations showed a minute and a half of warning would have been the minimum for the Tupolev to safely take evasive action.

The usual practice in such cases is to order an altitude change of 1,000 feet (300 meters) so the two planes could pass at different heights, he said. a descent speed of 1,000 feet per minute would have meant instructions needed to have been issued 90 seconds beforehand, Schlegel added.

According to him, the reconstruction so far showed the Tupolev began evasive action 30 seconds before the collision.

Skyguide officials on Friday dismissed Schlegel's claims, saying that the figures he cited were not mandatory guidelines.

Investigators in Germany were also studying the torn and tangled recordings in the four black boxes from the two aircraft, crash investigator Alex Thiel said in Braunschweig.

The four partly crushed boxes had been difficult to open without damaging them, Thiel said. Investigators found the magnetic tapes inside the boxes were badly stretched and cut in the crash. Thiel said the shreds were being spliced together.

All airliners carry two black boxes: One is a flight-data recorder and the other records cockpit conversations.