NTSB Chair Jennifer Homendy said there are 56 containers aboard containing hazardous materials, including corrosives, flammables and lithium ion batteries. She said some containers were breached and a sheen was identified in the water that will be dealt with by authorities. She said the voyage data recorder has been recovered.
Homendy said the investigation could take 12 to 24 months but that the NTSB will not hesitate to issue urgent safety recommendations during that time. She said a preliminary report should be released in two to four weeks.
"It's a massive undertaking for an investigation," Homendy said. "It's a very tragic event."
NTSB gives update on investigation into Baltimore bridge collapse
According to an unclassified memo from the federal Cybersecurity and Infrastructure Security Agency, federal officials are also monitoring about 1.8 million gallons of fuel inside the container ship Dali for its "spill potential." But a U.S. official familiar with the matter told CNN "lots would have to go wrong" for that amount of fuel to spill.
The Department of Homeland Security has also deemed the water near the crash site unsafe for divers, according to a memo obtained by ABC News.
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The concerns come after the vessel struck Baltimore's Francis Scott Key Bridge Tuesday morning, causing a near-total collapse.
Two bodies were recovered from the water Wednesday, according to Maryland State Police. Four others are still missing and presumed dead.
RELATED: Baltimore bridge death toll: Recovery efforts to resume for 6 construction workers presumed dead
An elite Coast Guard team is examining 13 damaged containers, "some with Centers for Disease Control and Prevention [CDC] and/or hazardous materials [HAZMAT] contents," the CISA memo said. The team is also analyzing the ship's manifest to determine what was on board and if any materials could pose a health risk, the source said.
"There is minor sheening on-scene. Booming has been ordered and is staged but will not be placed until search and rescue and dive operations are complete," the document said. "The amount of potential oil spill is 1.8M gallons of marine grade diesel."
Imagery from underwater drones show "an abundance of twisted metal and debris" from the collapsed bridge, making it unsafe for divers to enter the frigid water to search for the six missing construction workers, the DHS assessment added.
RELATED: What we know about the missing workers as recovery efforts resume after Baltimore bridge collapse
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One truck and trailer have been recovered, and one vehicle remains hanging from the metal structure, according to DHS.
With the Port of Baltimore closed to maritime traffic, transportation officials expect backups to rail and truck freight shipments as cargo shifts along the eastern seaboard, the document said.
RELATED: Baltimore bridge collapse and port closure send companies scrambling to reroute cargo
Questions over previous incidents involving ship and management company
As officials worked to investigate the collapse Tuesday, questions emerged over previous issues with the ship and its management company.
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The ship was briefly held at the Port of San Antonio in Chile on June 27, 2023, when an inspector found that the pressure gauges for the vessel's heating system were "unreadable," a spokesperson for the Chilean Navy said.
Ships managed by the Synergy Marine Group have been involved in at least three deadly incidents since 2018 in Australia, Singapore and the Philippines, according to officials in those countries.
RELATED: A list of major US bridge collapses caused by ships and barges
In 2018, a member onboard of a vessel managed by Synergy in Australia was killed in an accident involving the ship's personnel elevator, according to a report from the Australian Transportation Safety Bureau.
In 2019, an officer on a Synergy-registered vessel in Singapore was reported missing after "likely (falling) overboard while performing inspection or cleaning jobs," according to a report by the Transport Safety Investigation Bureau of Singapore's Ministry of Transport.
RELATED: Officials stopped traffic onto Baltimore's Key Bridge before collapse: 'These people are heroes'
In 2023, at least one sailor was killed when a Synergy Marine-managed tanker collided with a dredging ship in the Philippines, causing it to capsize, according to an incident report from the Philippines Coast Guard.
CNN contributed to this post.