The cargo ship that caused the Baltimore bridge to collapse was carrying hazardous materials, the National Transportation Safety Board (NTSB) said in a press briefing. 

NTSB Chair Jennifer Homendy said there were 56 containers aboard, identified with hazardous materials, including corrosives, flammables and lithium ion batteries. 

She said that some of the containers were breached and there was sheen on the waterway that will be dealt with by authorities. 

Homendy told the media that the ship's voyage data recorder has been recovered.

She said that the probe into the crash could take 12 to 24 months but added that the NTSB will not hesitate to issue urgent safety recommendations during that period. 

In fact, a preliminary report should be released in two to four weeks, she confirmed.

"It's a massive undertaking for an investigation," Homendy said. "It's a very tragic event."