The FBI has launched a criminal probe targeting the container ship that crashed into a major road bridge in Baltimore last month, collapsing it and killing six people, US media reported Monday.

The Washington Post described it as a "criminal investigation," and cited two unnamed US officials as saying the probe will look "at least in part" at whether the crew knew the ship had serious systems problems when it left the port.

In a statement to AFP, the Federal Bureau of Investigation said that it does not confirm or deny investigations as a matter of policy.

But it did say its agents were aboard the Singapore-flagged Dali container ship, which remains pinned beneath the wreckage of the Francis Scott Key Bridge nearly three weeks after the disaster.

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

The bridge, a major transit route into the busy city and port of Baltimore, was destroyed in seconds on March 26 when the Dali lost power and drifted into a support column.

The ship had managed to issue a Mayday call in the moments before the collision which gave police time to stop traffic on the bridge, likely saving lives.

But an eight-man construction crew repairing potholes on the bridge could not be reached in time and plummeted with the tons of concrete and twisted steel into the cold waters below.

Two workers were rescued alive -- one briefly hospitalized, but the other uninjured. The bodies of two more have been found, while four remain missing, believed to still be pinned beneath the wreckage.

Shipping in and out of Baltimore -- one of the United States' busiest ports -- has been halted, with the waterway impassable due to the sprawling wreckage.

The authorities hope that removing the bridge -- by cutting it into smaller sections and lifting them out -- will help rescuers recover all the victims' bodies and eventually reopen the crucial shipping lane.

The National Transportation Safety Board has also opened an investigation into the disaster.