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.
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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.
Video shows Baltimore's Francis Scott Key Bridge collapse after ship strike
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."
In the evening, Col. Roland L. Butler Jr., superintendent for Maryland State Police, announced that the search and rescue mission was transitioning to one of search and recovery. He also said the search was being put on pause and divers would return to the site at 6 a.m. Wednesday, when challenging overnight conditions are expected to improve.
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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.
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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.
Rescuers pulled two people out of the water. One person was treated at a hospital and discharged hours later.
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Five submerged vehicles have also been found in the water, Baltimore City Fire Chief James Wallace told CNN.
Authorities have been able to find three passenger vehicles, a cement truck and a fifth vehicle submerged in the water using infrared and side-scan sonar technology, according to Wallace.
"It looked like something out of an action movie," Baltimore Mayor Brandon Scott said, calling it "an unthinkable tragedy."
Given the vessel's massive weight, it struck the bridge support with significant force, said Roberto Leon, a Virginia Tech engineering professor.
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"The only way the post can resist it is by bending," Leon said. "But it cannot absorb anywhere near the energy that this humongous ship is bringing. So it's going to break."
Last June, federal inspectors rated the 47-year-old bridge in fair condition. But the structure did not appear to have pier protection to withstand the crash, experts said.
6 presumed dead following Francis Scott Key Bridge collapse
"If a bridge pier without adequate protection is hit by a ship of this size, there is very little that the bridge could do," Leon said.
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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."
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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.
Impacts from the collapse
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.
The collapse is sure to create a logistical nightmare for months, if not years, along the East Coast, shutting down ship traffic at the Port of Baltimore, a major shipping hub. The accident will also snarl cargo and commuter traffic.
Baltimore bridge collapse aftermath
"Losing this bridge will devastate the entire area, as well as the entire East Coast," Maryland state Sen. Johnny Ray Salling said.
Transportation Secretary Pete Buttigieg, speaking at a news conference near the site, said it was too soon to estimate how long it will take to clear the channel, which is about 50 feet (15 meters) deep.
"I do not know of a bridge that has been constructed to withstand a direct impact from a vessel of this size," he said.
Carnival Cruise Line said Tuesday that it is temporarily moving its Baltimore operations to Norfolk, Virginia following the Francis Scott Key Bridge collapse.
Carnival Legend is scheduled to return from its current voyage on Sunday, March 31. Officials said guests will return to Norfolk on Sunday and be bussed back to Baltimore.
The next seven-day Carnival Legend trip scheduled for March 31 will operate out of Norfolk.
Royal Caribbean International's Vision of the Seas is set to return to Baltimore on April 4, however no official changes have been made.
"We are deeply saddened by the tragedy and collapse of the Francis Scott Key Bridge and extend our heartfelt prayers to all those impacted. We are closely monitoring the situation, and our port logistics team is currently working on alternatives for Vision of the Seas' ongoing and upcoming sailings," said a spokesperson for Royal Caribbean.
'It felt like an earthquake'
According to a Coast Guard memo obtained by ABC News, a harbor pilot and an assistant aboard the cargo ship reported the power issues that prompted multiple alarms on the bridge of the vessel and loss of propulsion.
Synergy Marine Group - which manages the ship, called the Dali - confirmed the vessel hit a pillar of the bridge at about 1:30 a.m. while in control of one or more pilots, who are local specialists who help guide vessels safely into and out of ports. The ship is owned by Grace Ocean Private Ltd.
Synergy said all crew members and the two pilots on board were accounted for, and there were no reports of any injuries.
The ship was moving at 8 knots, roughly 9 mph (14.8 kph), the governor said.
PHOTOS: Francis Scott Key Bridge in Baltimore collapses after cargo ship rams into support column
Jagged remnants of the bridge could be seen jutting up from the water's surface. The on-ramp ended abruptly where the span once began.
Donald Heinbuch, a retired chief with Baltimore's fire department, said he was startled awake by a deep rumbling that shook his house for several seconds. "It felt like an earthquake," he said.
He drove to the river's edge and couldn't believe what he saw.
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Baltimore Governor Wes Moore discusses latest updates after 6 presumed dead in bridge collapse
"The ship was there, and the bridge was in the water, like it was blown up," he said.
The bridge spans the Patapsco River at the entrance to a busy harbor, which leads to the Chesapeake Bay and Atlantic Ocean. Opened in 1977, the bridge is named for the writer of "The Star-Spangled Banner."
Vessel traffic into and out of the Port of Baltimore -- the 9th largest port in the U.S. -- is suspended until further notice, however, officials said the port is still open for truck transactions.
President Joe Biden said he planned to travel to Baltimore and intends for the federal government to pick up the entire cost of rebuilding.
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"This is going to take some time," Biden said.
Where was the ship going?
The Dali was headed from Baltimore to Colombo, Sri Lanka, and flying under a Singapore flag, according to data from Marine Traffic. The container ship is about 985 feet (300 meters) long and about 157 feet (48 meters) wide, according to the website.
Inspectors found a problem with the Dali's machinery in June, but a more recent examination did not identify any deficiencies, according to the shipping information system Equasis.
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The most recent inspection listed for the Dali was conducted by the U.S. Coast Guard in New York in September. The "standard examination" didn't identify any deficiencies, according to the Equasis data.
Danish shipping giant Maersk said it had chartered the vessel.
The bridge, which is part of Interstate 695, opened in 1977 and crosses the Patapsco River in Baltimore's harbor. It is a vital artery, that along with the Port of Baltimore, is a hub for shipping on the East Coast.
Last year, the Port of Baltimore handled a record 52.3 million tons of foreign cargo worth $80 billion, according to the state.
The head of a supply chain management company said Americans should expect shortages of goods from the effect of the collapse on ocean container shipping and East Coast trucking.
"It's not just the port of Baltimore that's going to be impacted," said Ryan Petersen, CEO of Flexport.
The collapse, though, is not likely to hurt worldwide trade because Baltimore is not a major port for container vessels, but its facilities are more important when it comes to goods such as farm equipment and autos, said Judah Levine, head of research for global freight booking platform Freightos.
Detours and logistics
Drivers are being told to avoid the I-695 southeast corridor and use I-95 and I-895 as alternatives.
"I-695 is being detoured southbound at exit 43 (Peninsula Expressway) and northbound at Exit 2 (Route 10)," Wiedefeld said.
ABC News and The Associated Press contributed to this story.