GENEVA - Rescue workers used a helicopter and dogs Thursday to search for eight people still missing in a Swiss Alpine valley a day after a muddy rockslide barreled through a village on the Italian border.

Images from the scene showed a trail of destruction left by a river of mud and stone. An alarm system went off in time to allow for the evacuation of about 100 residents in the village of Bondo, 130 kilometers (80 miles) north of Milan.

The slide Wednesday morning sent about 4 million cubic meters (140 million cubic feet) of rocks and mud crashing down the mountain, causing an impact equivalent to 3.0 on the Richter scale, senior police official Andrea Mittner said.

Police in Graubuenden canton (state) said Thursday that they haven't been able to reach eight people who may have been in the Bondasca valley at the time of the slide — citizens of Germany, Austria and Switzerland, none of them children.

Mittner described the missing people as "Alpinists and walkers." He said a Swiss army helicopter searched the valley during the night, but found nothing.

On Thursday, workers began searching with dogs but didn't immediately find anyone. A helicopter equipped with a device that can locate cellphones also was being sent up. Around 120 people were involved in the operation — police, firefighters, troops and others.

"These people may have been in the disaster area at the time of the event," he told reporters in the nearby town of Stampa. "We hope this was not the case, but it is possible that they had an accident."

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"We don't know where exactly they are missing," he added. "The area is around 5 kilometers (3 miles) long."

Thursday afternoon, police received a separate, unverified report that a group of another five or six people could be missing — but spokeswoman Chiarella Piana of Graubuenden police said the group had turned up safely in Italy.

Markus Walzer of Graubuenden police said the local alarm system was put in place after a similar mudslide in 2012. He said the weather in the area had been good in recent days and the cause of the mudslide wasn't immediately known.

Mittner said Bondo would remain sealed off until at least Friday morning. It wasn't immediately clear when residents might be able to return home.