The US has opened a criminal investigation into what occurred on a cargo ship before it crashed into Baltimore's Francis Scott Key Bridge and caused it to collapse last month, according to a person familiar with the probe.

Federal Bureau of Investigation agents early Monday boarded the Singapore-flagged container ship, which remains disabled on the Patapsco River, the bureau said in a statement.

"The FBI is present aboard the cargo ship Dali conducting court authorized law enforcement activity," the FBI said. "There is no other public information available and we will have no further comment."

The Dali lost power and struck the bridge on March 26, killing six workers doing repairs on the span and bringing a halt to cargo traffic at the Port of Baltimore, where shipments of coal, cars and other goods have been disrupted. The container ship and workers on it have remained in place since.

The criminal investigation is examining what kind of failures occurred on the ship, the circumstances leading up to the crash and whether any federal laws may have been broken, according to the person familiar with the matter, who asked not to be identified speaking about information that isn't public.

Attorneys representing the Singapore-based ship owner, Grace Ocean Ptd Ltd., earlier this month filed a petition jointly with Synergy Marine Pte Ltd., which was operating the ship. They claim the collapse of the bridge was "not due to any fault,...