A rescue operation is underway to locate six construction workers who were on Baltimore’s Francis Scott Key Bridge when a container ship struck it, collapsing into the Patapsco River early Tuesday morning, authorities said.

The Associated Press reported that the ship lost power, and operators had made a distressing call just moments before it crashed into one of the bridge’s supports, causing the structure to partially break away and plunge into the river and atop the vessel, where it caught fire and black smoke was seen rising from it.

“Never would you think that you would see, physically see, the Key Bridge tumble down like that. It looked like something out of an action movie,” Baltimore Mayor Brandon Scott said in a statement obtained by AP, calling it “an unthinkable tragedy.”

Kevin Cartwright, the Baltimore Fire Department’s director of communications, described the incident as a “developing mass casualty event.” He also confirmed that several vehicles, including a tractor-trailer truck, were parked on the bridge when the collision happened.

Reports indicate that the construction crew worked on the bridge when it collapsed. So far, officials said two individuals have been rescued from the waters, while six remain unaccounted for.

Maersk, a major shipping company, stated that the container vessel involved in the collision was under its charter and was carrying its cargo as it left the port around 1:30 a.m. Tuesday, USA Today reported. Additionally, no Maersk personnel were aboard the ship at the time of the incident.

The Dali vessel was operated by Synergy Group and owned by Singapore-based Grace Ocean Pte. Marine Traffic, a ship tracking service, confirmed that the ship was slated to arrive in Sri Lanka in April.

“We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected,” Maersk said in a statement obtained by USA Today. “We are closely following the investigations conducted by authorities and Synergy, and we will do our utmost to keep our customers informed.”

The Port of Baltimore primarily serves as a hub for importing and exporting automobiles and other goods along the East Coast. According to Bloomberg, more than 40 ships are stuck inside the port, causing an indefinite shutdown until further notice.

Maryland Gov. Wes Moore has stated that the collision is still in its early stages of investigation, describing it as an “accident” and ruling out any indication of terrorism.