The bodies of two people have been recovered from a red pickup truck, which was under water where the Baltimore bridge collapsed.
Eight construction workers were on the bridge when a ship struck it, plunging them into the waters below.
Two of the workers were rescued on the day, but the search continues for the other four - all presumed dead.
Salvage crews are working to address hazardous materials and accident investigators are on the scene.
Four of the six victims of the bridge collapse have been named so far. At a news conference on Wednesday, Maryland State Police identified Alejandro Hernandez Fuentes, 35, and Dorlian Ronial Castillo Cabrera, 26, as the two workers recovered by divers from inside the truck. Mr Fuentes is originally from Mexico and Mr Cabrera is from Guatemala.
But divers are no longer able to navigate the waters safely because of concrete and debris found in the river, police said.
They are now using sonar scans and believe that vehicles that may contain other bodies are "encased in superstructure and concrete" that came down from the bridge, an official said.
Two other missing victims, who are presumed dead, have also been named: Miguel Luna, originally from El Salvador, and Maynor Suazo Sandoval, a Honduran citizen.
Mexico's Ministry of Foreign Affairs said earlier that two of its citizens were presumed dead - one of whom is now identified as Mr Fuentes - while another was rescued from the water.
One person who was in hospital after being pulled out of the water was released, officials said late Wednesday.
First responders spent hours on Tuesday searching the waters of the Patapsco River for the six construction workers, who were working on potholes on the bridge at about 01:30 (05:30 GMT) when the ship crashed into the bridge. The US Coast Guard called off the search around sunset, saying cold water temperatures and hours gone by meant the workers were presumed dead.
Officials have pledged to find the bodies of the men for their relatives.
"We've got to give these families closure," Wes Moore, governor of Maryland, told reporters on Wednesday, adding that air, land and water resources had been devoted to the search of the victims.