The Dali container ship experienced a near-perfect storm of calamities before it struck Baltimore's Francis Scott Key Bridge in March, killing six people.
But Tuesday's preliminary report by federal safety investigators leaves many questions unanswered.
For example, the National Transportation Safety Board (NTSB) described four power failures that beset the 300-metre ship.
The reasons for three of them are yet to be explained, including two that occurred right before the crash.
Seven weeks on, 21 sailors — most of them from India — remain stranded on board the Dali due to the ongoing investigation.
Even as explosives were used to destroy part of the bridge on the ship's hull on Monday, the crew were not allowed to disembark.
So what have authorities uncovered so far? Here's what we know and don't know about the problems that unfolded on the ship before the disaster.
What do we know about the trapped crew?
The 21 crewmen, made of 20 Indians and a Sri Lankan national, are still on board.
Some worry they’ll be held personally liable for the disaster.
"While some crew members are coping, morale has understandably dipped," two unions representing the seafarers said in a statement.
Reverend Mark Nestlehutt, president and executive director of the New York-based Seamen's Church Institute, said he and others boarded the Dali about a week after the crash to provide a "compassionate ear" to the crew.
"Everybody was trying to make the best out of a tragic situation," Reverend Nestlehutt said.
"At that point, the only real question for the seafarers was when they might be able to go home."
Crew members have expressed concerns about their phones being seized by federal investigators, Reverend Nestlehutt said.
Darrell Wilson, a spokesperson for Synergy Marine, the Dali's Singapore-based management company, said the phones have been replaced with new ones, and Synergy Marine said in a statement in early April that the crew had unlimited use of the ship's satellite communications to stay in touch with family.
But Reverend Nestlehutt and unions say the new phones lack the important personal information that's on the old phones, such as contacts, family photos and banking apps for transferring money back home to their families.
The two unions representing crew members, the Singapore Maritime Officers' Union and the Singapore Organisation of Seamen, called for the "swift return" the phones in a statement.
The unions said the men also suffered emotional distress from witnessing the crash and have an "unfounded fear of personal criminal liability".
"The criminalisation of seafarers based solely on their position on board a vessel during an incident is a growing concern," said Mary Liew, general secretary for the officers' union.
Reverend Nestlehutt also said the crewmen are concerned that continuing to be detained on the ship could imperil future visas to the US or for the crew members’ children.
How many times did the ship lose power?
The Dali suffered four blackouts in roughly 10 hours: two the day before the crash and two in the minutes before.
The first came after a crew member mistakenly closed an exhaust damper during maintenance, causing one of the ship's diesel engines to stall, according to the safety report.
A backup generator came on, but insufficient fuel pressure soon caused that generator to fail, resulting in a second blackout. It's unclear why the fuel pressure dropped.
In response, crew members changed the ship's electrical configuration, which is considered routine.
They switched from one transformer and breaker system that had been in use for several months to another configuration.
The third outage struck hours later after the ship had left the Port of Baltimore and was navigating the Patapsco River.
Electrical breakers unexpectedly tripped, causing a power loss. The Dali's diesel engine automatically shut down because its cooling pumps lost power.
The pilot's dispatcher called the police and notified the coastguard of the power loss. Crew members momentarily restored electricity by manually closing the breakers.
Then more breakers tripped, causing another outage as the ship approached the bridge. The crew restored power, but it was too late.
One of the pilots ordered the rudder turned, but since the main engine remained down, there was no propulsion to assist with steering, the report said.
The ship struck one of the bridge's main supports, causing the span to collapse.
What may have caused the blackouts?
The reason behind one power outage is known — the crew's failure to close the exhaust damper.
The reason for the second blackout remains unclear, and the two instances of the tripped breakers appear to be complete mysteries.
Jennifer Homendy, chair of NTSB, told a congressional committee on Wednesday, local time, that the first two blackouts were "mechanically distinct" from the second two outages.
She said switching out the breakers following the first two outages "may have affected operations the very next day on the accident voyage".
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Stefano Brizzolara, a professor of ocean engineering at Virginia Tech, pointed out that the crew switched to a configuration that hadn't been used for several months.
He compared this move to putting on a pair of shoes that you haven't worn in a long time that have become "hard and stiff".
"Every time you switch to electrical components that are not normally used, the risk of failure increases a little bit," he said.
But changing up a ship's power configuration is fairly routine, even if it's been several months, said Neil Gallagher, a professor at the Webb Institute, a school that teaches naval architecture and marine engineering.
"It shouldn't change anything," Professor Gallagher said.
"Transformers are a pretty benign item. It's just coils of wire. It doesn't have moving parts. It doesn't have a whole lot that breaks on it. So, it's not normally the source of issues like this."
What else could have been done?
ProfessorGallagher added that the mistake with the damper would have caused some concern but not enough to raise major worries before leaving port.
"They went for 10 hours and had no problems," he said.
"And in all fairness, the breakers that shut down when they were underway were not the same as what happened when the engine lost power because they closed the damper."
ProfessorGallagher said cargo ships do not routinely lose power.
"This is kind of a perfect storm of events that happened at just the absolute worst possible time," he said.
ProfessorBrizzolara said the crew seemed to have reacted to the power failures promptly.
"I don't know if they could do more," he said.
"These things can happen, unfortunately. Machines are prone to fault. And yes, there are redundancies onboard of ships. But even redundant systems can fail."
Bradley Martin, a former US Navy captain and a senior policy researcher at the RAND Corporation, said the limited space in many ports increases the risks of these types of disasters.
"Anybody who's ever been around ships knows that machinery fails at inopportune times," Mr Martin said.
"And being so close to infrastructure like this means there's no good way of reacting quickly enough."
Thomas McKenney, a University of Michigan professor of naval architecture and marine engineering, said the tragedy in Baltimore raises questions about whether most cargo ships have enough safeguards.
"I think the relevant question here is how reliable and redundant should systems be onboard ships, especially understanding that ships have gotten larger and larger over time," Professor McKenney said.
"Shipping remains by far the most efficient way to transport goods. So it's really having the right balance between cost efficiency without compromising safety."
AP/ABC
Posted, updated
