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Two UPS workers died at a shipping hub at Ontario International Airport on Monday morning after a tractor-like vehicle tipped over and crushed them.

The Ontario Fire Department notified the California Division of Occupational Safety and Health at 7:21 a.m. about the double fatality.

“According to the outside source, a tug tipped over and crushed the two employees,” Cal/OSHA spokesperson Luke Brown said. Tugs are tractor-like vehicles used to tow heavy shipment.

The airport is a major shipping hub. In 2016, it underwent an expansion to double its production, according to the Press Enterprise.

Read the full story on LATimes.com.