Divers have recovered the remains of two of the six missing workers tossed into Baltimore Harbour from a highway bridge that collapsed after being rammed by a faltering cargo freighter.

The bodies of Alejandro Hernandez Fuentes, from Mexico, and Dorlian Ronial Castillo Cabrera, from Guatemala, were pulled from the Patapsco River on Wednesday (local time).

They were found a day after the massive container ship lost power and its ability to manoeuvre before ploughing into a support pylon of the Francis Scott Key Bridge, knocking most it into the water below.

A Maryland State Police official said the red ute containing the bodies of Fuentes and Cabrera was found in about eight metres of water near the midsection of the fallen bridge.

Further efforts to recover the bodies of four people still missing have been suspended due to increasingly treacherous conditions.

“We are now moving from a recovery mode to a salvage operation,” Maryland State Police superintendent Colonel Roland L. Butler Jr said.

“Because of the superstructure surrounding what we believe were the vehicles and the amount of concrete and debris, divers are no longer able to safely navigate and operate around that.

“We have exhausted all search efforts.”

Butler said that based on sonar scans, officials believed the vehicles of the workers who are still missing are “encased in the superstructure and concrete” of the bridge.

The men were part of a crew filling potholes on the bridge when it fell. They were declared late on Tuesday (local time) to be presumed dead, 18 hours after the crash.

Butler said there was “no definitive timeline” to complete the salvage phase. Divers will return to the site after that.

“The sonar simply said they cannot get to that area because it was fully encased in the superstructure,” he said.

“Once that salvage effort takes place and that superstructure is removed, those same divers are going to go back out there and bring those people closure.”

‘Hold all traffic’: Moments that saved lives

The collapse of the bridge forced an indefinite closure of the Port of Baltimore, one of the busiest on the US eastern seaboard, handling more automobile and farm equipment freight.

Earlier on Wednesday, federal investigators examined the cargo ship while details emerged of the intense efforts to save lives in the minutes before the steel span collapsed.

“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 recording offered a glimpse of how authorities scrambled before the crash sent eight bridge repair workers on the night shift to their deaths in the frigid black waters.

The Singapore-flagged Dali 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 and created a traffic quagmire for Baltimore and the densely populated region.

The bridge collapse could cost insurers billions of dollars in claims, with one analyst putting the cost at as much as $US4 billion ($6 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. They returned to the vessel on Wednesday to interview the crew, other survivors and emergency responders, NTSB chair Jennifer Homendy said.

She said the investigation was “a massive undertaking”.

“We have an amazing team of individuals who are focused on very specific areas of expertise and so I have no doubt that we will be able to pull this together in hopefully 12 to 24 months,” Homendy said.

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

The four presumed dead are all migrants from Central America.

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 US Army engineers to remove the debris, he said.

The wreck drew attention to the vessel’s safety record, but Gautier said, the ship had a “fairly good safety record”.

Of the ship’s 4700 cargo containers, 56 hold 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.

-with AAP