Behind the scenes of the Baltimore bridge collapse

In the dark, early morning hours of 26 March, US transportation secretary Pete Buttigieg's phone rang - and he immediately knew he had a problem.

"If my phone rings in the middle of the night, it's not a good thing," he recalled.

The Dali, a massive 948ft (289m) cargo ship had slammed into Baltimore's iconic Francis Scott Key Bridge, sending the 1.5-mile (2.4km) bridge into the cold waters of the state's Patapsco River.

"It wasn't immediately clear what we were dealing with," Buttigieg told the BBC in an interview. "How many people had been impacted, and how much of the bridge had been destroyed."

Six men, all of a road crew working on the bridge, were killed in the incident, which left the Dali - still afloat - stuck under huge chunks of shredded metal and concrete. Another man was pulled from the icy water, seriously injured.

A new documentary from BBC One - available on iPlayer on 5 December - reveals new details of the investigation into the crash, including a possible cause: a cable that shook loose, disrupting the ship's power supply and causing an initial blackout minutes before it struck the bridge.

After that, investigators believe they discovered a lack of fuel pressure to the generators may have created power fluctuations that tripped breakers and caused the ship to go dark again, with no time to change course before slamming into the bridge.

• Watch on iPlayer: Why Bridges Collapse: The Baltimore Disaster
• Lost power, a mayday call and the crash that brought down a Baltimore bridge

Previously unseen bodycam footage taken after the collapse shows first responders and officials struggling to come to grips with the enormity of what they faced in the confusing hours after the crash.

"Key Bridge is down. It was last reported that there are at least several vehicles in the water," an officer can be heard saying. "And several people still uned for."

In the aftermath of the collapse, an estimated 45,000 tonnes of debris blocked the Patapsco, a 700ft (213m) wide and 50ft (15m) deep and economically vital shipping channel.

Recovering the bodies and removing the twisted, broken steel and concrete from the channel - plus moving the Dali - fell to a massive team including US Army, Navy and Coast Guard , as well as Maryland authorities and specialist private firms.

The woman who led the effort, Colonel Estee Pinchasin, the Army Corps of Engineers Baltimore district commander, said that she had "never seen anything of that magnitude before".

"Everything [was] mangled up on top and around," she said. "You had these big, large spans that were just laying in the water. You see four-inch steel that's been bent... how can you even start to think about the force":[]}