Multiple alarms sounded on board the doomed Dali cargo ship in the minutes before it smashed into Baltimore's Key Bridge, new black box audio data revealed. 

The NTSB said it received six hours of audio from the ship's voyage data stretching from midnight until 6am. The vessel impacted moments before 1.30am.

Marcel Muise, the NTSB lead investigator in the case, said Wednesday that alarms began blaring on the ship at 1.24am.

At 1.26am, the ship's pilot requested urgent help from nearby tug boats, and at 1.27am he ordered the ship's anchor be dropped. 

The exact cause of the catastrophe that has left at least six people dead is still under investigation, with some form of mechanical failure speculated as footage also showed the ship's lights turning on and off several times in the moments before. 

The new data was revealed at a press conference on Wednesday, where NTSB Chief Jennifer Homendy confirmed all 23 crewmembers - including two temporary pilots intended to only safely steer the vessel out of port - were safe. 

Over 4,600 cargo units were on board the ship, 56 of which contained hazardous materials. Homendy said some of the hazardous cargo was breached, but did not indicate it posed a public safety threat. 

After Muise offered details of alarms sounding off in the moments before impact, Honendy said the data falls short of what investigators would expect if they were investigating a similar catastrophe involving an aircraft.