A labeling band on a single signal wire was the cause of the electrical blackout that led to the collision of the containership Dali with Baltimore's Francis Scott Key Bridge in March 2024, which resulted in the deaths of six highway workers, according to a report from the National Transportation Safety Board (NTSB) released on Tuesday.
The investigation revealed that the wire-label banding prevented the wire from being fully inserted into its terminal block spring-clamp gate, leading to a weak connection in the electrical system of the 984-foot vessel. When the wire disconnected, a high-voltage breaker opened unexpectedly. This triggered a series of events that resulted in the loss of propulsion and steering as the Singapore-flagged containership was leaving Baltimore Harbor.
“The Dali is almost 1,000 feet long, equivalent to the height of the Eiffel Tower, with extensive electrical wiring and numerous connections,” said NTSB Chairwoman Jennifer Homendy. “Locating this single wire was like searching for a loose rivet on the Eiffel Tower.”
The blackout happened around 0129 local time on March 26, 2024, cutting power to critical systems such as steering gear pumps, the fuel oil flushing pump, and cooling water pumps for the main engine. As a result, the Dali started to drift toward the Key Bridge's Pier 17. Despite attempts by the pilots and bridge team to change the vessel's course, the loss of propulsion made their efforts futile.
When the ship hit the southern pier supporting the central span of the bridge, a significant part of the structure collapsed into the Patapsco River. Sections of the pier, deck, and truss spans fell onto the bow and front container bays of the vessel.
Six out of seven members of a road maintenance crew were killed in the collapse. One highway worker survived but suffered serious injuries, while an inspector escaped unscathed. Only one of the 23 people aboard the Dali had a minor injury.
The NTSB concluded that the probable cause of the accident was the “loss of electrical power (blackout) due to a loose signal wire connection stemming from the improper installation of wire-label banding.” This led to the vessel losing propulsion and steering near the bridge. It was also noted that the lack of measures to reduce the bridge's vulnerability to impacts from ocean-going vessels contributed to the collapse and the resulting fatalities. Additionally, there was inadequate communication to alert the highway workers to evacuate the bridge.
The investigation pointed out further safety concerns. The vessel's main engine was set up to shut down when the cooling water pressure was low—a design that, while it met standards at the time, posed risks when the cooling pump lost power. Investigators also identified issues with the flushing pump, which was incorrectly being used as a fuel service pump for diesel generators without a backup.
"Like all the accidents we look into, this one was preventable," Homendy stated. "Taking NTSB recommendations from this investigation to heart can help avoid similar tragedies in the future."
The investigation also noted that if infrared thermal imaging had been used as part of the Dali's preventive maintenance program, the loose wire could have been detected earlier.
The vulnerability of the bridge to large vessel strikes was another critical factor. The Key Bridge opened in 1977, and since then ships have significantly increased in size. The Dali was 984 feet long, compared to the Blue Nagoya, which caused only minor damage when it made contact with the same bridge back in 1980.
During the Dali investigation, the NTSB extended its inquiry beyond Baltimore and contacted 30 owners of major bridges over navigable waters across the United States. The agency requested that these bridge owners assess how at risk their structures are from impacts of much larger ocean-going vessels today and, if needed, create risk-reduction strategies.
The directive requires bridge owners to apply established AASHTO (American Association of State Highway and Transportation Officials) guidelines regarding vessel-collision design, estimate the risk of collapse, and consider practical countermeasures like fendering, structural upgrades, and traffic management systems. The Key Bridge collapse is being viewed as a wake-up call for all aging bridges in the U.S. that are exposed to maritime traffic.
The NTSB acknowledged that prompt actions by the pilots of the Dali, shore dispatchers, and the Maryland Transportation Authority helped to prevent a greater loss of life by stopping traffic on the bridge.
The damage to the Dali was estimated to exceed $18 million, with cargo damages still undetermined. Repair costs for the bridge are projected to be between $4.3 billion and $5.2 billion, with completion expected by late 2030. Over 34,000 vehicles, including 10% of trucks and all vehicles carrying hazardous materials that are prohibited from using Baltimore’s tunnels, will now have to take longer detours.
Grace Ocean and Synergy Marine Group, the companies that own and manage the vessel, thanked the NTSB for its professionalism and rigor during the investigation. They stated they would carefully review the findings with their technical teams and legal counsel.
Eight crew members from the Dali are still in the United States to assist with the investigation, with four granted permission to visit their families in early December.
The NTSB has made 18 new safety recommendations to various organizations, targeting issues from vessel redundancy and electrical maintenance to bridge protection and emergency communications:
In March 2025, the NTSB made urgent recommendations regarding bridge vulnerability to vessel strikes:
A summary of the actions taken on Tuesday, including the probable cause, findings, and recommendations, can be found on ntsb.gov. The full investigation report will be released in the upcoming weeks.