NTSB: Single Loose Wire Blamed for Baltimore Bridge Collapse That Killed Six

Lead

U.S. investigators from the National Transportation Safety Board (NTSB) concluded on Tuesday that a single loose signal wire aboard the Singapore‑flagged container ship Dali precipitated two electrical blackouts that disabled propulsion and steering, causing the March 26, 2024 collision with the Francis Scott Key Bridge in Baltimore. Six highway construction workers on the bridge were killed when part of the span collapsed; investigators say the ship lost electrical power after a faulty terminal connection. The NTSB described the defect as a design/assembly failure of a metal sleeve and a labeling band that prevented a wire from fully seating. Agency staff, working with the shipbuilder, inspected thousands of wires before isolating the one that failed.

Key Takeaways

• The collision occurred on March 26, 2024, when the Singapore‑flagged container ship Dali struck the Francis Scott Key Bridge over the Patapsco River, Maryland.
• The NTSB identified the “probable cause” as a loose signal wire connection that led to two catastrophic blackouts and loss of propulsion and steering on the Dali.
• Investigators traced the failure to a metal sleeve design flaw that allowed a wire labeling band to overtake the sleeve and prevent a proper terminal insertion.
• Thousands of wires were examined in concert with HD Hyundai Heavy; the faulty wire was ultimately isolated after extensive testing and analysis.
• Six construction workers died when part of the bridge collapsed; investigators concluded they might have survived if warned sooner.
• Officials found 1 minute and 29 seconds elapsed between police being notified to block bridge traffic and the time of impact, a window investigators say could have allowed workers to move to a safer section.
• NTSB published an explanatory animation on its X account illustrating how the labeling band and sleeve interaction caused the disconnect.

Background

The Francis Scott Key Bridge is a major highway structure spanning the Patapsco River and carries heavy commuter and commercial traffic into Baltimore. Large container ships transit the same channel to reach the Port of Baltimore; maritime and highway safety depend on coordinated navigation, vessel power reliability and timely communications. Modern large container ships rely on centralized electrical systems for propulsion, steering, alarms and bridge automation; a loss of electrical power can rapidly degrade maneuverability.

Industry practices include mechanical fastening, labeled wiring and protective sleeves to assure secure electrical terminations; when those components fail, a single point of failure can cascade into larger system outages. The Dali is Singapore‑flagged and was built with components supplied or approved through global shipbuilding supply chains, where design tolerances and assembly practices vary by vendor. Past maritime incidents show that small hardware faults — when combined with operational stressors — can produce outsized consequences.

Main Event

On March 26, 2024, while six highway construction workers were on the bridge, the Dali experienced two sudden blackouts that eliminated propulsion and steering until the ship struck a support of the Francis Scott Key Bridge. The NTSB’s post‑accident reconstruction showed the vessel lost course following the power failures and collided with a critical pier, causing a section of the bridge to collapse. Emergency services responded; six workers did not survive the collapse.

During the technical investigation, NTSB engineers traced the blackouts to an intermittently connected signal wire within a control circuit. Video animation and wiring mockups released by the agency showed how a labeling band migrated and overtook the metal crimp sleeve intended to hold the conductor in the terminal, preventing a full insertion and eventual loss of contact under vibration and load.

NTSB Chair Jennifer Homendy characterized the search for the fault as extraordinarily difficult, noting that investigators and HD Hyundai Heavy tested vast numbers of wire assemblies and components before identifying the defective termination. The agency determined the loose connection was a probable cause element that triggered the sequence of failures leading to the collision and collapse.

Analysis & Implications

Technically, the incident highlights how a seemingly minor hardware or assembly issue at a single electrical terminal can propagate into total system loss on vessels that depend on integrated electrical control. Redundancy in power and steering systems can mitigate, but not always prevent, failures when a primary control path is compromised. The Dali’s two successive blackouts suggest that the loose connection caused intermittent contact before a full disconnect, complicating detection by crew before critical loss of control.

From a regulatory and industry perspective, the finding will likely prompt reviews of wiring sleeve designs, labeling practices and quality‑assurance protocols across shipyards and suppliers. Ship operators, classification societies and equipment manufacturers may be urged to reexamine terminal designs and to mandate post‑assembly mechanical verification or enhanced vibration testing for similar terminations.

On the safety management side, the limited time window — 1 minute and 29 seconds from police notification to impact — draws attention to emergency alerting mechanisms for bridge crews and rapid traffic control. Investigators concluded that timely warning could have allowed workers to relocate to a part of the bridge that did not collapse; that gap raises questions about dispatcher procedures, automated alarms and the chain of notification between vessel, port authorities and roadway incident managers.

Comparison & Data

The table above contrasts the 2024 Key Bridge catastrophe with the 2007 I‑35W bridge collapse as a reminder that structural loss of life can stem from both maritime accidents and structural design failures. While the Key Bridge event is rooted in a maritime electrical/assembly issue that led to impact, historical bridge disasters often involve design, material fatigue or maintenance lapses. The contrast underscores that mitigation strategies must be cross‑domain — addressing both maritime safety and roadway incident response.

Reactions & Quotes

Officials and investigators emphasized the painstaking nature of tracing the root cause and the potential for procedural changes.

“It’s like finding a needle in the haystack,”

Jennifer Homendy, NTSB Chair

Homendy used the simile to explain the difficulty of detecting a single failed wire among thousands of terminations, and to stress the collaborative effort with HD Hyundai Heavy to test many components. The NTSB also framed its determination using formal language of causation.

“Probable cause: loss of electrical power due to a loose signal wire connection,”

National Transportation Safety Board (NTSB)

The agency’s statement summarized its technical conclusion and signaled possible follow‑on actions for industry standards and recommendations. Local and state transportation officials have said they will review notification and traffic‑control protocols in light of the 1 minute 29 second timeline identified by investigators.

Unconfirmed

• Whether identical sleeve and labeling configurations are present on other vessels of the same class and whether they pose an immediate systemic risk remains under review.
• Whether alternative onboard alarms or automatic collision‑avoidance logic could have prevented the strike has not been established.
• Claims that faster notification alone would have certainly saved all six workers are not confirmed; investigators state it “could have” provided time to move to a safer span but outcome certainty is unknown.

Bottom Line

The NTSB’s conclusion that a single loose signal wire triggered a catastrophic chain of events elevates the importance of small, hardware‑level safeguards across global shipbuilding supply chains. Technical fixes — better sleeve designs, stricter assembly verification and enhanced vibration testing — are likely to follow, along with regulatory scrutiny of wiring terminations on new and existing vessels.

Equally important are procedural changes for emergency communications between maritime operators, port authorities and roadway incident managers: investigators identified a very short window in which faster notification plausibly would have allowed workers to reach a safe section of the bridge. Policymakers and industry leaders should treat this event as both a hardware failure and a systems‑of‑systems failure requiring coordinated remedies.

Sources

• DW (Deutsche Welle) — international news report summarizing NTSB findings
• NTSB — official U.S. federal accident investigation agency (official investigation)
• NTSB on X — agency posts including the animation used to illustrate the wiring failure (official social media)
• HD Hyundai Heavy — shipbuilder/manufacturer (corporate)