At least 9 dead in France avalanche, police say NEW: Three of those killed are British, the UK Foreign Office says Nine people are confirmed dead and four people are still missing, police say 28 climbers are caught by falling snow high in the Mont Blanc range, police say (CNN) -- A summer climbing expedition in the French Alps turned to tragedy Thursday when a six-foot-thick wall of snow came crashing down on two groups of climbers, leaving at least nine dead, French police said. Two people caught in the avalanche on the Mont Blanc massif were found alive in the snow afterward, but at least four people remained missing, police said. Nine people were taken to a hospital with slight injuries. A UK Foreign Office spokesman said three of those killed were British. Two groups of climbers were caught in the avalanche on the north side of Mont Maudit in the Mont Blanc range, near Chamonix on the French-Italian border, police said. A total of 28 climbers in the two groups were connected by ropes, authorities in Annecy, France, said. A mountain rescue unit was called out at 5:25 a.m. after the avalanche was triggered when a sheet of ice dislodged. The route the climbers were taking up Mont Maudit is very popular with alpinists tackling the mighty Mont Blanc, said Alexis Hennebelle of Radio Mont Blanc. The climbers were very unlucky to be caught in such a catastrophic avalanche, he told CNN. Local newspaper Le Dauphine said it was the deadliest accident in the French Alps since 2003. French Interior Minister Manuel Valls visited Chamonix Thursday afternoon to see the rescue efforts for himself. Speaking to reporters, he said the Mont Blanc range is, sadly, often the scene of dramas, but this is a particularly unfortunate incident. Valls paid tribute to the efforts of the emergency workers, French and Italian, who have searched in perilous conditions of snow, ice and wind since early morning for those who are still missing. "They've shown their professionalism, their sense of duty, to bring help in difficult conditions," he said. Valls said it was too soon to establish any responsibility for what had happened, and asked people to wait for the results of an investigation. French newspaper Le Figaro quoted a Chamonix official as saying weather conditions had been good on Thursday morning, giving the climbers no clue that a deadly avalanche might be unleashed. CNN meteorologist Mari Ramos said changes in temperature and heavy rainfall over the past week in the area may have created conditions in which sheets of ice like the one that fell are formed. Mont Blanc is the highest mountain in Western Europe at nearly 15,800 feet. Mont Maudit, some 1,000 feet lower and the third highest peak in the range, seems to have lived up to its name, which can be translated as hateful or cursed. CNN's Richard Allen Greene contributed to this report.