Investigators probing the disappearance of 6-year-old Swiss twins said Wednesday they have been unable to confirm witness sightings of the girls _ leaving them as uncertain as ever about their fate.

Hope of finding Alessia and Livia Schepp alive has faded since last week, when a letter from their father surfaced, saying he had killed them but leaving no other clues. Matthias Kaspar Schepp committed suicide in Italy after sending the grim letter, which said the girls were "resting in peace."

Investigators in France, Italy and Switzerland have been trying to trace their father's path in the days after he left his home near Lake Geneva with the girls on Jan. 30. He then traveled to southern France, Corsica and Italy _ either with or without his daughters.

On Wednesday, investigators from the three countries huddled in the southern French city of Marseille to coordinate their efforts.

Officials said no possibilities had been ruled out, including that the girls are still alive.

"The inquiry is still very active," Marseille Prosecutor Jacques Dallest told reporters.

Witnesses have come forward to say they saw the girls on the ferry from southern France to Corsica, as well as on the island. One person in Corsica reported seeing the girls with their father and an unknown woman on Feb. 1. But investigators said they aren't certain Alessia and Livia ever made it to the island.

Such accounts "haven't necessarily been corroborated by material evidence," Dallest said. Lausanne Prosecutor Pascal Gillieron added that the ferry cabin where their father stayed had been cleaned several times before investigators examined it, leaving behind few clues.

Schepp often carried a small voice recorder with him. But the Lausanne prosecutor denied reports investigators had found it.

The girls' mother, Irina Lucidi, was quoted in Swiss media as saying the girl's father, from whom she was separated, might have picked up an accomplice at Lyon airport, based on clues from his cell phone.

The Lausanne prosecutor said investigators were looking at the possibility Schepp was accompanied by another person, but he noted that the airport was on Schepp's route and he didn't make a detour to pass by it.

Lucidi was in Corsica in recent days to help investigators probe sites on the French island, where the family vacationed together in happier times.

Investigators turned up nothing during a search of a well on a nature reserve on the tiny Finocchiarola islands off Corsica, a police official said. He spoke on condition of anonymity because he was not authorized to speak to media. Investigators also probed an abandoned lime kiln in the area.