"Hold all traffic on the Key Bridge. There's a ship approaching that just lost their steering," someone said on police radio minutes before the 1.30am crash on Tuesday.
While voices were heard discussing next steps, including alerting any work crews to leave the bridge, one broke through to say: "The whole bridge just fell down!"
The audio was carried by Broadcastify, an open-source audio streaming service.
The recording offered a glimpse of how authorities scrambled before the crash into the Francis Scott Key Bridge sent six bridge repair workers on the night shift to their deaths in the frigid black waters of Baltimore Harbor.
The Singapore-flagged Dali had reported a loss of power before impact and dropped anchor to slow the vessel, giving authorities barely enough time to halt traffic on the bridge and likely prevent greater loss of life.
The disaster closed the Port of Baltimore, one of the busiest on the US eastern seaboard, and created a traffic quagmire for Baltimore and the densely populated region.
A bridge work crew was repairing potholes when the ship ploughed into the bridge. (AP PHOTO)
The bridge collapse could cost insurers billions of dollars in claims, analysts said, with one putting the cost as much as $US4 billion ($A6 billion), which would make the tragedy a record shipping insurance loss.
Investigators from the US National Transportation Safety Board recovered the data recorder after boarding the ship late on Tuesday and returned to the vessel on Wednesday to interview the ship's crew, other survivors and emergency responders, NTSB chair Jennifer Homendy said.
A highway team was due to examine the remains of the 2.6km-long bridge for answers to how and why the container ship smashed into a support pillar.
Rescuers pulled two workers from the water alive on Tuesday, and one was hospitalised.
The six presumed dead included immigrants from Mexico, Guatemala and El Salvador, according to the Mexican Consulate in Washington.
One from Mexico was rescued, and two were missing, said Mexico President Andrés Manuel Lopez Obrador.
The bridge work crew was repairing potholes on the road surface when the Dali ploughed into the pylon as it left Baltimore for Sri Lanka.
The search for the missing workers' remains have been hampered by cold, steady rain. (AP PHOTO)
A trestled section of the bridge almost immediately crumpled into the water, sending vehicles and workers into the 50-foot-deep waters at the mouth of the Patapsco River.
Divers were looking for their remains in a cold, steady rain.
"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," Maryland Governor Wes Moore said.
The US Coast Guard priorities were to restore the waterway for shipping, stabilise the vessel and extricate it, Vice Admiral Peter Gautier said at a White House briefing.
"The real critical thing here is that, as you know, a portion of the bridge remains on the bow on that ship," he said.
The Coast Guard would work with the US Army engineers to remove the debris before the vessel was moved, he said.
The wreck drew attention to the vessel's safety record, but Gautier said, the ship had a "fairly good safety record".
The disaster closed the Port of Baltimore, one of the busiest on the US eastern seaboard. (EPA PHOTO)
Of the ship's 4700 cargo containers, 56 held hazardous materials but there was no threat to the public, he said.
Two containers went overboard during the crash but they did not contain hazardous materials.
The ship was carrying more than 5.7 million litres of fuel oil, he said.
The NTSB's Homendy said the agency would examine the ship's data and also whether contaminated fuel played a role in the ship's power loss.
The Port of Baltimore handles more car freight than any other US port - more than 750,000 vehicles in 2022, according to port data, as well as container and bulk cargo ranging from sugar to coal.
Still, economists and logistics experts said they doubted the port closure would unleash a major US supply chain crisis or major spike in the price of goods, due to ample capacity at rival shipping hubs along the eastern seaboard.
