BALTIMORE — The FBI has opened a criminal investigation into the Baltimore bridge collapse, sources familiar with the matter told the BBC's US partner CBS.

The investigation will examine the circumstances leading up to the incident, in which six people were killed.

The Francis Scott Key Bridge fell on March 26 after it was struck by a large container ship called the Dali. Federal agents have been present at the site since, the sources said.

In a statement on Monday, a spokesperson for the Department of Justice (DoJ) said the "FBI is present aboard the cargo ship Dali conducting court authorized law enforcement activity".

No further information will be shared publicly on the investigation at this time, the spokesperson said.

Erek Barron, the US Attorney in Maryland who is aiding in the FBI investigation, said his office would not comment on the existence of a probe.

But he said "the public should know, whether it's gun violence, civil rights abuse, financial fraud, or any other threat to public safety or property, we will seek accountability for anyone who may be responsible".

The FBI investigation is separate from an ongoing probe by the National Transportation Safety Board (NTSB).

Most of the 47-year-old bridge collapsed when it was struck by Dali — a 948ft (289m) cargo ship — at around 01:28 local time, plunging eight men who were working on bridge maintenance at the time into the Patapsco River.

The bodies of three have since been recovered and three others are presumed to be dead by authorities. Two others were rescued shortly after the bridge fell.

A salvage mission that includes the US Army Corps of Engineers, the Navy, the Coast Guard and Maryland authorities is still ongoing to recover parts of the bridge, as well as to find the bodies of the remaining victims.

Investigators also have recovered the data recorder of the Dali ship, which they hope may explain the catastrophic power failure that caused it to go off course and hit the bridge.

Officials said the ship had been moving at eight knots, or roughly 9mp (15 km/h).

Jennifer Homandy, the head of the NTSB, has said one of the factors under investigation is whether contaminated fuel played a role in the ship losing power.

A mayday signal was sent when the ship was approaching the bridge, authorities had said, which gave authorities time to stop cars driving onto the bridge.

The collapse led to the closure of the Port of Baltimore, one of the busiest in the US and a key hub for the transport of vehicles, though two alternative channels have since been opened as the salvage operation continues. — BBC