The U.S. workplace safety watchdog is investigating the circumstances around the collapse during Friday night's storm of an Amazon.com Inc building in Illinois in which six workers died, an official at the U.S. Department of Labor said on Monday.

The U.S. Occupational Safety and Health Administration (OSHA) has six months to complete its investigation, issue citations, and propose monetary penalties if violations of workplace safety and/or health regulations are found, Scott Allen, a U.S. Department of Labor regional director for public affairs, said via email. He added that compliance officers have been on site since Saturday.

Six workers were killed when the Amazon warehouse in Edwardsville, Illinois, buckled  under the force of a devastating storm, police said. A barrage of tornadoes 2 ripped through six U.S. states, leaving a trail of death and destruction at homes and businesses stretching more than 200 miles (322 km).

Amazon spokeswoman Kelly Nantel said the company would cooperate with the investigation. "OSHA investigates all workplace fatalities and we are supporting them," she said.

Responding to a reporter, Illinois Governor J. B. Pritzker said officials are investigating the condition that the warehouse was in before it gave way.

"Already there has been an effort to determine some of the challenges... if there were any structural issues, what exactly the storm's trajectory was coming in and affecting the various pieces of the building," Pritzker said. "That's already begun and is ongoing."

The National Weather Service said a tornado hit the area between 8:28 p.m. and 8:32 p.m. local time on Friday, intensifying rapidly as it struck the Amazon warehouse. With estimated peak winds of 150 miles per hour (241 kph), the force was so severe that the roof was ripped off and 11-inch (28-cm) thick concrete walls longer than football fields fell in on themselves.

At least 45 Amazon employees made it out safely.

The company has three facilities in Edwardsville: the delivery station hit by the storm, as well as a fulfillment center and a sorting station. The delivery station opened in July 2020 to prepare orders for last-mile delivery to customers.

The site, a compound comprised of a few buildings, has been under heavy private and police security through the weekend, accessible only to relief teams and some Amazon and government officials. The remains of the destroyed warehouse were mostly dismantled, reduced to rubble.