Maryland's governor has warned of a "very long road ahead" to recover from the loss of Baltimore's Francis Scott Key Bridge as the Biden administration approved $US60 million in immediate federal aid after the deadly collapse.
Meanwhile US Army Corps engineers were moving the largest crane on the eastern seaboard to help remove the wreckage of the bridge, Governor Wes Moore said, so work to clear the channel and reopen the key shipping route could begin.
The machine, which can lift up to 1000 tonnes, was expected to arrive on Thursday evening, and a second crane with a 400-tonne capacity might arrive on Saturday.
Maryland was "deeply grateful" for the $US60 million ($A92 million) in federal money, Moore said at a news conference on Thursday.
Moore promised "the best minds in the world" were working on plans to clear the debris, move the cargo ship that rammed into the bridge from the channel, recover the bodies of the four remaining workers presumed dead and investigate what went wrong.
The channel must be cleared before the key shipping route can be reopened. (AP PHOTO)
"Government is working hand in hand with industry to investigate the area, including the wreck, and remove the ship," said Moore, who said the quick aid was needed to "lay the foundation for a rapid recovery".
President Joe Biden has pledged the federal government would pay the full cost of rebuilding the bridge.
"This work is not going to take hours. This work is not going to take days. This work is not going to take weeks," Moore said.
"We have a very long road ahead of us."
The devastation left behind after the powerless cargo ship struck a support pillar early on Tuesday is extensive.
Divers recovered the bodies of two men from a truck in the Patapsco River near the bridge's middle span on Wednesday, but officials said they had to start clearing the wreckage before anyone could reach the bodies of four other missing workers.
Federal investigators boarded the Dali to recover its data recorder and to interview the crew. (AP PHOTO)
Based on sonar scans, the vehicles appear to be encased in a "superstructure" of concrete and other debris.
National Transportation Safety Board officials boarded the ship, the Dali, to recover its data recorder and to interview the captain and crew.
Investigators shared a preliminary timeline of events before the crash, which federal and state officials have said appeared to be an accident.
The victims, who were part of a construction crew fixing potholes on the bridge, were from Mexico, Guatemala, Honduras and El Salvador.
At least eight people initially went into the water when the ship struck the bridge column, and two of them were rescued on Tuesday, officials said.
The crash caused the bridge to break and fall into the mouth of the Patapsco River within seconds, blocking shipping lanes and forcing the indefinite closure of the Port of Baltimore, one of the busiest on the US eastern seaboard.
The huge vessel was carrying nearly 4700 shipping containers, 13 of which were destroyed. (AP PHOTO)
Authorities had just enough time to stop vehicle traffic, but did not get a chance to alert the construction crew.
The Singapore-flagged Dali, which is managed by Synergy Marine Group, was headed from Baltimore to Sri Lanka.
Synergy on Thursday extended sympathies to the victims' families and the people of the Baltimore region, noting it was co-operating with investigators.
The huge vessel, nearly as long as the Eiffel Tower is tall, was carrying nearly 4700 shipping containers, 56 of them with hazardous materials inside.
Thirteen of those were destroyed, officials said.
However, industrial hygienists who evaluated the contents identified them as perfumes and soaps, according to the Key Bridge Joint Information Center, which said there was no immediate threat to the environment.
AP and Reuters