BALTIMORE — A cargo ship lost power and rammed into a major bridge in Baltimore early Tuesday, destroying the span in a matter of seconds and plunging it into the river in a terrifying collapse that could disrupt a vital shipping port for months. Six people were missing and presumed dead.

The ship’s crew issued a mayday call moments before the crash took down the Francis Scott Key Bridge, enabling authorities to limit vehicle traffic on the span, Maryland’s governor said.

The ship struck one of the bridge’s supports, causing the structure to collapse like a toy. A section of the span came to rest on the bow of the vessel, which caught fire.

With the ship barreling toward the bridge at “a very, very rapid speed,” authorities had just enough time to stop cars from coming over the bridge, Maryland Gov. Wes Moore said.

“These people are heroes,” Moore said. “They saved lives last night.”

The crash happened in the middle of the night, long before the busy morning commute on the bridge that stretches 1.6 miles and was used by 12 million vehicles last year.

The six people still unaccounted for were part of a construction crew filling potholes on the bridge, said Paul Wiedefeld, the state’s transportation secretary.

A senior executive at the company that employed the workers said Tuesday afternoon that they were presumed dead, given the water’s depth and the length of time since the crash.

Jeffrey Pritzker, executive vice president of Brawner Builders, said the crew was working in the middle of the bridge when it came down. No bodies have been recovered, and rescuers continued the search into the late afternoon.

“This was so completely unforeseen,” Pritzker said. “We don’t know what else to say. We take such great pride in safety, and we have cones and signs and lights and barriers and flaggers.”

Jesus Campos, who has worked on the bridge for Brawner Builders and knows members of the crew, said he was told they were on a break and some were sitting in their trucks when the bridge went down.

“I know that a month ago, I was there, and I know what it feels like when the trailers pass,” Campos said. “Imagine knowing that is falling. It is so hard. One would not know what to do.”

Rescuers pulled two people out of the water. One person was treated at a hospital and discharged hours later. Multiple vehicles also went into the river, although authorities did not believe anyone was inside.

“It looked like something out of an action movie,” Baltimore Mayor Brandon Scott said, calling it “an unthinkable tragedy.”

A police dispatcher put out a call just before the collapse saying a ship had lost its steering and asked officers to stop all traffic on the bridge, according to Maryland Transportation Authority first responder radio traffic obtained from the Broadcastify.com archive.

One officer who stopped traffic radioed that he was going to drive onto the bridge to alert the construction crew. But seconds later, a frantic officer said: “The whole bridge just fell down. Start, start whoever, everybody … the whole bridge just collapsed.”

On a separate radio channel for maintenance and construction workers, someone said officers were stopping traffic because a ship had lost steering. There was no follow-up order to evacuate, and 30 seconds later the bridge fell and the channel went silent.

From 1960 to 2015, there were 35 major bridge collapses worldwide due to ship or barge collision, according to the World Association for Waterborne Transport Infrastructure.