Dramatic dashcam video shows the moment a crane came crashing down at Google's new campus in Seattle on Saturday. Four people were killed, including college student Sarah Wong and iron worker Travis Corbet, and four others were hurt.

Seattle Mayor Jenny Durkan said it is a miracle that a mother and her baby girl survived the collapse. The 25-year-old mother and her four-month-old daughter made it out of a crushed car with only minor injuries.

Witnesses said it sounded like an earthquake when a crane collapsed onto Google's new, under-construction headquarters Saturday, hitting six cars on one of Seattle's busiest streets.

An image taken from dashcam video of a deadly construction crane collapse in Seattle Saturday, April 27, 2019. Natalie Williams

"It was so loud that you felt it," said witness James Etherington. "The ground shook when it hits and it was like, oh my God!"

Two ironworkers inside the crane were killed, along with two people in separate cars.

Bruce Cashmere and his son took a time-lapse video of the crane just before it fell. He said it was being disassembled by workers.

"The next thing, kabang, down she come," said Cashmere. "And I knew what it was."

There is no known cause yet, but wind gusts at the time of the accident were as high as 23 miles per hour, reports correspondent Janet Shamlian.

With construction booming, Seattle has around 60 active cranes, more than any other U.S. city, and roughly double the number in Portland or Chicago. It's become known as the nation's crane capital.

The Department of Labor reports that between 2011 and 2015, there was an average of 44 crane-related deaths each year in the U.S. But none of them was in Seattle.

Authorities say they've opened an investigation into four companies over the incident.

"We are really at the beginning of an investigation that will likely take months," said Tim Church, of the Washington State Department of Labor and Industries. He promised to learn the cause, so it won't happen again.

"No one should ever have to go to work and not come home from their job," Church said.

The site where this happened is managed by the real estate firm Vulcan, which said it sends its deepest sympathies to the families who lost loved ones.

Vulcan's general contractor, GLY, said it is cooperating with investigators.