Four people have died and four others have been injured after a crane collapsed onto a street in Seattle, slamming into a Google office building under construction and trapping people inside cars.

The collapse took place on Mercer Street in downtown Seattle on Saturday afternoon. Four people were already dead when rescuers arrived at the scene, Seattle Fire Chief Harold Scoggins told a press conference.

The death toll includes two crane operators and two people who died in separate cars after the crane wreckage crumpled their roofs. Three people were taken to hospital and one was treated at the scene.

Overall, six cars were hit by flying debris, the fire chief said.

A building at the intersection of Fairview/ Mercer has taken significant damage. pic.twitter.com/SUq10eiI1D — Evan Bush (@evanbush) April 27, 2019

A four-month-old girl is among the persons injured in the collision. The little girl, together with her 25-year-old mother, was taken to hospital in a stable condition. A 28-year-old man remains in a serious condition, KOMO News reported.

Photos from the scene show part of the crane collapsed on the side of a sprawling Google office complex under construction. The construction of the four-building complex began in 2017 and was supposed to include towers as high as 15 stories.

The other part of the crane fell onto the street, crashing down onto the cars parked nearby.

"Half of it was flying down sideways on the building. The other half fell down on the street, crossing both lanes of traffic," the Seattle Times reported, citing a witness.

Seattle Times journalist Evan Bush wrote on Twitter that some of the cars were "pancaked" while construction workers scrambled to help the survivors to crawl out of the mangled vehicles.

Alan Van Rosendael said he heard a large crash “like thunder” then saw a plume of dust. When he could see what happened, he saw construction workers trying to pry someone out of a car. Several cars were “pancaked,” he said. pic.twitter.com/eZQ5hDEB6N — Evan Bush (@evanbush) April 27, 2019

A powerful gust of wind reportedly triggered the collapse, witnesses told the paper, while police did not elaborate on the cause of the incident. Traffic around the scene was shut down until Sunday, with Seattle Department of Transportation advising drivers to opt for alternative routes.

Seattle has been a magnet for tech companies like Amazon, Google and many others who have built their offices there, turning the city into a giant construction ground. Seattle boasts around 60 construction cranes, more than any other city in the US.

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