US federal safety investigators have recovered the black box from the freight ship that crashed into a Baltimore bridge as rescuers searched  dark waters for the remains of six construction workers lost in the bridge collapse.

A highway team also will examine the twisted remains of the Francis Scott Key bridge as they try to determine how and why the Dali container ship smashed into a pillar of the 2.6-kilometre span in early morning darkness on Tuesday.

Investigators from the US National Transportation Safety Board recovered the data recorder after boarding the ship late on Tuesday, board chair Jennifer Homendy said.

They will interview the ship’s crew and other survivors, she said.

Tweet from @SkyNews

The disaster forced the indefinite closure of the Port of Baltimore, one of the busiest on the US eastern seaboard, and created a traffic quagmire for Baltimore and the surrounding region.

Rescuers pulled two construction workers from the water alive on Tuesday. One was hospitalised.

The disaster may be the worst US bridge collapse since 2007 when 13 people died in Minneapolis.

Victims from multiple countries

The six presumed to have perished included immigrants from Mexico, Guatemala and El Salvador, the Mexican consulate in Washington DC said.

One of the missing has been named as father of three Miguel Luna, originally from El Salvador.

A second victim was Honduran citizen Maynor Suazo Sandoval, whose death was confirmed by his brother to NBC News, BBC reports.

Seven Brawner Builders executive vice-president Jeffrey Pritzker told CNN the workers who were on the Francis Scott Key Bridge on the night of the collapse were wonderful young men.

“They were doing a tough job. These guys were hard-working, wonderful people and now they’re gone,” he said

One survived, and the other six are presumed dead.

Pritzker didn’t know how the worker managed to survive, CNN reported, saying the survivor wasn’t ready to talk about his experience. He was “very, very upset”, injured and traumatised.

Officials said the eight on the bridge were part of a work crew repairing potholes on the road surface when the Singapore-flagged Dali, which was leaving Baltimore bound for Sri Lanka, ploughed into a support pylon.

A trestled section of the bridge almost immediately crumpled into the water, sending vehicles and workers into the river.

Search in darkness, mangled metal

The US Coast Guard said it was looking for the bodies 18 hours after they were thrown from the bridge into the frigid waters at the mouth of the Patapsco River.

“They are down there in darkness where they can literally see about a foot in front of them, they are trying to navigate mangled metal” in a place where people are presumed to have lost their lives,” Maryland Governor Wes Moore said at the scene.

Tweet from @Joy7News

Moore said at a Tuesday news briefing the bridge was up to code and had no known structural issues.

The 289-metre-long ship had reported a loss of propulsion shortly before impact and dropped anchor to slow the vessel, giving transportation authorities crucial minutes to halt traffic on the bridge before the crash.

That move likely prevented a higher death toll, authorities said.

It was unclear whether authorities also tried to alert the work crew ahead of the impact.

Contaminated fuel considered

Collection of data from the Dali would provide investigators with a timeline of what happened as soon as Wednesday, the NTSB’s Homendy said as she prepared to board the vessel.

The process will involve taking photos of the ship and the bridge, getting electronic logs and also interviewing first responders.

The agency will also examine whether contaminated fuel played a role in the ship’s power loss, Homendy said.

The Baltimore wreck drew attention to the vessel’s safety record.

The ship was involved in an incident in the port of Antwerp, Belgium, in 2016, hitting a quay as it tried to exit the North Sea container terminal.

A 2023 inspection in Chile found “propulsion and auxiliary machinery” deficiencies, according to data on the Equasis public website, which provides information on ships.

But Singapore’s Maritime and Port Authority said the vessel passed two separate foreign-port inspections in June and September 2023. It said a faulty fuel pressure gauge was rectified before the Dali departed port following its June 2023 inspection.

Video footage on social media showed the vessel slamming into the 47-year-old Key Bridge in darkness, the headlights of vehicles visible on the span as it crashed into the water and the ship caught fire.

All 22 crew members on the ship were accounted for, management company Synergy Marine Pte Ltd reported.

Tuesday’s disaster may be the worst US bridge collapse since 2007, when the I-35W bridge in Minneapolis plunged into the Mississippi River, killing 13 people.

-with AAP