As a controlled explosion rocked the Dali on Monday, nearly two dozen sailors remained on board, below deck in the massive ship's hull.

The simultaneous blasts sent pieces of Baltimore's once iconic Francis Scott Key Bridge into the dark waters of Maryland's Patapsco River, seven weeks after its collapse left six people on the bridge dead and the Dali marooned.

Authorities - and the crew - hope that the demolition will mark the beginning of the end of a long process that has left the 21 men on board trapped and cut off from the world, thousands of miles from their homes.

But for now, it remains unclear when they will be able to return home.

The Dali - a 948ft (289m) container ship - was at the start of a 27-day journey from Baltimore to Sri Lanka when it struck the Francis Scott Key Bridge, sending thousands of tonnes of steel and cement into the Patapsco. It left the ship stranded under a massive expanse of shredded metal.

A preliminary NTSB report found that two electrical blackouts disabled equipment ahead of the incident, and noted that the ship lost power twice in the 10 hours leading up to the crash.

The crew, made up of 20 Indians and a Sri Lankan national, has been unable to disembark because of visa restrictions, a lack of required shore passes and parallel ongoing investigations by the National Transportation Safety Board (NTSB) and FBI.

On Monday, the crew remained on board even as authorities used small explosive charges to deliberately "cut" an expanse of the bridge laying on the ship's bow.

Ahead of the controlled demolition, US Coast Guard Admiral Shannon Gilreath said that the crew would remain below deck with a fire crew at the ready.

"They're part of the ship. They are necessary to keep the ship staffed and operational," Adm Gilreath said. "They're the best responders on board the ship themselves."

While the ship is likely to be re-floated this week, it remains unclear when it will be able to make the 2 nautical mile (3.7km) journey to port.

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