Baltimore's biggest bridge collapses after being hit by a cargo ship, videos show. Mass casualty event declared.

• Baltimore's Francis Scott Key Bridge collapsed early on Tuesday morning.
• A livestream captured the moment a ship collided with one of the bridge's support beams.
• Authorities are calling it a "mass casualty event" and say 20 people went into the water.

Baltimore's Francis Scott Key Bridge collapsed on Tuesday morning after it was struck by a large container ship.

A video of the incident was posted early on Tuesday morning to X, formerly Twitter. In the video, the container ship the Dali is seen colliding with one of the bridge's support beams. Smoke is seen billowing from the ship before the bridge began crumbling.

"MAJOR BALTIMORE TRAFFIC ALERT: AVOID I-695 southeast corridor. I-695 Key Bridge collapse due to ship strike," the Maryland Transportation Authority said in an X post on Tuesday morning.

A representative for the Baltimore Police Department told ABC News that "at 1:35 a.m., Baltimore City police were notified of a partial bridge collapse, with workers possibly in the water, at the Francis Scott Key Bridge."

Emergency services teams, including divers and at least two helicopters, responded to the scene, per Baltimore County's police scanner in the hour and a half after the bridge's collapse.

Wes Moore, governor of Maryland, declared a state of emergency early on Tuesday.

He said he was working to "quickly deploy federal resources from the Biden Administration," per the BBC.

In a news conference later in the day, Moore said that the crew on the ship notified authorities they had lost power.

Moore told reporters that the ship was moving at a "very rapid speed" and that the call to the authorities stopped a greater disaster from occurring, with cars redirected away from the bridge.

He also said the bridge was "fully up to code" and had no structural issues.

Paul Wiedefeld, Maryland's transportation secretary, told reporters that there were workers on the bridge at the time repairing potholes, and that six people were unaccounted for.

The White House said it was "closely monitoring" the situation, and that there was no indication of any nefarious intent.

"Our hearts go out to the families of those who remain missing as a result of this horrific incident," a spokesperson told BI in a statement.

Secretary of Transportation Pete Buttigieg said on X that he had offered the DOT's support to Moore and Baltimore Mayor Brandon Scott.

The Baltimore Fire Department estimates that up to 20 people people went into the water, the BBC reported.

Baltimore Fire Department Chief James Wallace said at a press conference early Tuesday that two people had been recovered from the water. One refused service, while another was transported to a local trauma center "in very serious condition," he said.

The fire department had not made contact with the ship's captain yet, he said.

A livestream view of the area at around 3:00 a.m. local time showed the bridge's structure partially submerged in the harbor and in several pieces.

An unclassified Cybersecurity and Infrastructure Security Agency report said that the ship "lost propulsion" as it was leaving port, ABC News reported.

The crew notified officials that they had lost control and warned of a possible collision, the report said, per ABC News.

Structural engineer Ian Firth told the BBC that the heavy ship would have struck the bridge support backed by many thousands of metric tons.

"The support is a very, relatively, flimsy structure when you look at it, it's a kind of trestle structure with individual legs," he told the outlet. "So, the bridge has collapsed simply as a result of this very large impact force."

Barbara Rossi, an engineering professor at the University of Oxford, told BI: "According to what I could see online, the bridge has received a huge impact force on one of its supporting structures."

"The impacting force must have been immense," she said.

The Dali is owned by Grace Ocean, a Singapore-based firm. The firm confirmed in a statement on Tuesday morning that their vessel had struck one of the bridge's pillars, per TradeWinds.

"All crew members, including the two pilots, have been accounted for and there are no reports of any injuries. There has also been no pollution," read the firm's statement.

It had 22 crew aboard, the Maritime and Port Authority of Singapore said in a statement.

According to Grace Ocean, the vessel was bound for Colombo, Sri Lanka when the accident took place.

Maersk, the ship's charterer, confirmed that vessel company Synergy Group operates the ship. Maersk's spokesperson told BI in a statement that the company is "horrified" by the events, and added that no Maersk employees were on board.

"We are closely following the investigations conducted by authorities and Synergy," it added.

As of Tuesday morning, authorities warned mariners and drone operators to avoid the waters and nearby airspace.

Baltimore County Executive Johnny Olszewski, Jr. said in an X post that the authorities "are closely monitoring the ongoing situation at the Key Bridge."

"Our prayers remain with all those impacted," Olszewski wrote.

The Francis Scott Key Bridge opened in March 1977 as the final link in the Baltimore Beltway, according to the MDTA.

It cost $60.3 million to build and is 10.9 miles long, per the MDTA.