The bodies of two construction workers were found in the cold waters of Baltimore harbor Wednesday, trapped in their red pick-up truck after a giant cargo ship slammed into the bridge they had been filling potholes on, causing a thunderous collapse.

Maryland police announced the grim discovery at a press conference, adding that sonar shows what they believe are more vehicles trapped within the concrete and twisted steel debris of the Francis Scott Key Bridge.

Six of the eight-man construction crew are believed to have been killed, with four bodies yet to be found.

Warning that it was not safe for divers to try to penetrate the wreckage, police told a press conference that they were shifting to a salvage operation, removing the superstructure and then sending divers back in to recover the rest of the bodies.

"Based on sonar scans, we firmly believe that the vehicles are encased in the superstructure and concrete that we tragically saw come down," Colonel Roland Butler, the superintendent of Maryland's state police, told a press conference.

Federal investigators also gave a detailed timeline of the tragedy based on preliminary findings from the ship's voice data recorder.

Marcel Muise, lead investigator for the National Transportation Safety Board, told a separate press conference that the container ship Dali, about 1,000 feet (300 meters) long and piled high with cargo, left dock at 12:39 am Tuesday en route to Asia.

At 1:24, alarms began sounding on the ship with indications of power trouble and the pilot soon radioed the port authority that the vessel was headed for the bridge, requesting tug boats.

The call for help was heard by two Maryland Transportation Authority units on the bridge because of the roadwork, and they shut down all lanes of traffic, likely saving lives.

Muise told reporters that at 1:29 the voice data recorder captured "sounds consistent with the collision."

Nearly the entire steel structure -- crossed by tens of thousands of motorists each day -- collapsed within seconds, cascading over the bow of the ship, blocking one of the busiest US trading ports.

There was no chance to evacuate the eight workers filling potholes on the interstate directly above the oncoming ship.

Butler named the two victims found Wednesday as Alejandro Hernandez Fuentes, a 35-year-old who had lived in Baltimore but was originally from Mexico, and his 26-year-old colleague Dorlian Ronial Castillo Cabrera, who lived in the suburb of Dundalk but came from Guatemala.

They were found in 25 feet of water, he said.

Two others were pulled from the water alive in the moments after the collapse early Tuesday. One was uninjured, while the second was released from hospital Wednesday, Butler said.

Four more workers are presumed dead, vanished into the swirling currents and crumpled tangle of wrecked girders and pylons.

- 'Hard-working' men -

The vessel, which remained entangled in the debris Wednesday, was "stable," Coast Guard Vice Admiral Peter Gautier told reporters at the White House, adding that the mostly Indian crew remained on board and were "very much engaged" in the investigation.

The NTSB said that at the time of the crash there were 23 crew on board including the two pilots.

The agency said the ship held 56 containers of hazardous materials, some of which were breached after the bridge fell, leaving a sheen on the water.

Gautier insisted the ship did not present an environmental danger. Two other containers -- of the total 4,700 -- were lost overboard, he said.

Officials said the missing workers were from El Salvador, Guatemala, Honduras and Mexico.

"They are all hard-working, humble men," said Jesus Campos, a colleague of the eight workers, all employed by contractor Brawner Builders.

One of those now presumed dead was father-of-three Miguel Luna, according to the nonprofit Casa, which serves immigrant communities.

Luna, from El Salvador, had left for work at 6:30 pm on Monday and never returned, Casa said.

His wife, Maria del Carmen Castellon, told Telemundo 44 that she was "devastated" by the wait for any information.

- Busy harbor blocked -

The ship had passed two overseas inspections in 2023, the maritime authority for Singapore, where the ship is flagged, said Wednesday, adding that a fault monitor gauge was fixed in June.

The Port of Baltimore is the ninth-busiest major US port in terms of both foreign cargo handled and foreign cargo value, and is directly responsible for more than 15,000 jobs, supporting almost 140,000 more.

The effect on supply chains "clearly will not be trivial," US Transport Secretary Pete Buttigieg said, adding it was "too soon" to know when the port might reopen.

"Rebuilding will not be quick, or easy, or cheap," he cautioned.