"At this moment people are on the scene still searching," Hollande said, speaking alongside German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy in Seynes-les-Alpes, the town nearest to the crash site. Most of the plane's victims were German and Spanish.
    "This is a true tragedy, and the visit here has shown us that," Merkel said.
    Hollande promised that French investigators would do everything to determine the crash's cause.
    Helicopters surveying the plane's scattered debris lifted off at daybreak for a look at the craggy ravine while emergency crews hiked through snow and rain over the steep, rocky terrain to the high-altitude crash site. In all, more than 600 rescue workers and aviation investigators were in the area, French officials said.
    The crash left pieces of wreckage "so small and shiny they appear like patches of snow on the mountainside," Pierre-Henry Brandet, the Interior Ministry spokesman, said after flying over the debris field.
    Investigators were zooming in on two key minutes Tuesday - 10:30-10:31 a.m. - said Segolene Royal, a top government minister whose portfolio includes transport. From then on, air traffic controllers were unable to make contact with the plane.
    German Interior Minister Thomas de Maiziere told reporters in Berlin "there is no hard evidence that the crash was intentionally brought about by third parties."
    The plane, operated by Germanwings, a budget subsidiary of Lufthansa, was less than an hour from landing in Duesseldorf when it unexpectedly went into a rapid, eight-minute descent. The pilots sent out no distress call, France's aviation authority said.
    The four possible causes of any crash are human error, mechanical problems, weather, criminal activity or a combination of two or more. Investigators will use the cockpit voice and flight data recorders to map out and focus their work, said Alan E. Diehl, a former air safety investigator with the National Transportation Safety Board.
    "Both will point you in directions of what is critical," Diehl says. "Based on what you learn from the recorders, you might focus on key pieces of wreckage."
    Diehl says investigators will essentially work backward.
    "You're usually dealing with a jigsaw puzzle with many of the pieces missing," he says. "You start eliminating things that didn't happen."
    Lufthansa said two charter flights to France will be made available for family members who want to get as close as they can to the crash site. Locals in Seyne-les-Alpes offered to host the bereaved families due to a shortage of rooms to rent.
    Germanwings itself cancelled several flights Wednesday because some crews declared themselves unfit to fly after losing colleagues.
    "The management completely understands this, because we are a small family. Everyone knows everybody inside Germanwings, so it is a big shock for employees," said CEO Thomas Winkelmann.
    Three Americans, including a mother and daughter, were among the victims, the U.S. State Department said Wednesday.
    Winkelmann said the company had already contacted most families of the victims and was trying to reach the rest. He said victims included 72 German citizens, 35 Spaniards, two people each from Australia, Argentina, Iran, Venezuela and one person each from Britain, the Netherlands, Colombia, Mexico, Japan, Denmark, Belgium and Israel.
    Some could have dual nationalities, for Spain's government said 51 citizens had died in the crash.
    The victims included two babies, two opera singers, an Australian mother and son vacationing together, and 16 German high school students and two teachers returning from an exchange program in Spain.
    The principal of Joseph Koenig High School, Ulrich Wessel, called the loss of 16 of his students and two teachers - one who had just gotten married and another who was soon to be - a "tragedy that renders one speechless."
    "Nothing will be the way it was at our school anymore," he said.
    Paul Andrew Bramley, a 28-year-old from Britain, had been studying hospitality and hotel management in Lucerne and was flying to meet his mother before starting an internship on April 1.
    "He was the best son. He was my world," said his mother, Carol Bramley.
    In Spain, flags flew at half-staff on government buildings and a minute of silence was held in government offices across the country. Parliament canceled its Wednesday session.
    Barcelona's Liceu opera house held two minutes of silence at noon to honor two German opera singers - Oleg Bryjak and Maria Radner - who took the flight after performing at the theater last weekend.