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Nepal Earthquake: American David Chen Witnesses Temple's Collapse

David Chen, director of engineering at San Francisco firm Skycatch, witnessed parts of ancient Nepal temple Changu Narayan collapse on Tuesday.

An American watched as Tuesday's massive earthquake turned parts of an ancient Nepalese temple into "clouds of dust."

"It seemed like it would never stop," David Chen, 32, told NBC News. "It was extremely strong. It felt like the building was swaying back and forth."

Chen is director of engineering at San Francisco firm Skycatch, which is using drones to help map UNESCO heritage sites damaged in last month's 7.8-magnitude quakethat killed more than 8,000 people.

When the magnitude-7.3 quake struck on Tuesday, Chen was on a rooftop about to shoot drone footage of Changu Narayan, an ancient Hindu temple outside of Kathmandu.

"We were on the building so it was really scary," he said. "It definitely felt much larger than any of the aftershocks."

Chen managed to film the earthquake using his cellphone. His footage showed "a lot of clouds of dust" after a corner of the ancient temple crumbled, as well as "other houses of the villagers that were collapsing from that earthquake."

The temple is built in the two-tier pagoda style and contains statues and inscriptions more than 1,500 years old, according to Lonely Planet.

"At first I was just kind of in shock not knowing what to do," Chen told NBC News of the moment the quake struck. "There had been aftershocks but they had all been very quick, just a few seconds. But this one, it just seemed liked it would never stop. Eventually I just ran down the stairs and tried to get out of the house as fast as I could."