BALTIMORE (AP) — Investigators probing the March collapse of the Francis Scott Key Bridge in Baltimore said in a preliminary report Tuesday the cargo ship Dali experienced an electrical blackout about 10 hours before leaving the Port of Baltimore while undergoing maintenance.

The power outage was caused by a crewmember mistakenly closing an exhaust damper, causing the ship’s engine to stall, the report issued by the National Transportation Safety Board said. The ship lost power again and crashed into one of the bridge’s supporting columns shortly after leaving the port on March 26, which brought the bridge down in seconds.

A full investigation could take a year or more, the agency said.

The board launched its investigation almost immediately after the collapse, which sent six members of a roadwork crew plunging to their deaths. Investigators boarded the ship to document the scene and collect evidence, including the vessel’s data recorder and information from its engine room, according to board chair Jennifer Homendy. Investigators also interviewed the captain and crew members.

“Our mission is to determine why something happened, how it happened and to prevent it from recurring,” Homendy said at a news conference days after the disaster.

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According to the preliminary report, at 1:25 a.m. on March 26, when the Dali was a little over half a mile away from the bridge, a primary electrical breaker that fed most of the ship’s equipment and lighting unexpectedly tripped, causing the ship to lose electrical power and experience a blackout. The main propulsion diesel engine shut down after the pumps lost electrical power. The ship’s crew was able to restore power, then called for an assist from tug boats and the senior pilot ordered the ship’s anchor to be dropped.

A second blackout then occurred and a marine radio call was made to warn waterborne traffic. The ship then struck a main support pier on the bridge, causing it to collapse within seconds.

The ship, which was headed from Baltimore to Sri Lanka, issued a mayday alert with just enough time for police to stop traffic, but not enough to save the workers filling potholes on the bridge.

The last of the victims’ bodies was recovered last week.

On Monday, crews conducted a controlled demolition to break down the largest remaining span of the collapsed Francis Scott Key Bridge, a major step in freeing the grounded Dali container ship.

The board’s preliminary report released Tuesday likely includes a fraction of the findings that will be presented in its final report, which is expected to take more than a year.

Testing of the ship’s fuel did not reveal any concerns related to its quality, according to the preliminary report.

The FBI has also launched a criminal investigation into the circumstances leading up to the collapse.

Homendy said the National Transportation Safety Board investigation would probe all aspects of the crash, including what caused the ship’s power loss and whether it had been experiencing similar issues prior to the blackout.

Investigators also planned to review policies, training practices and other factors that could be relevant. And the design, engineering and condition of the bridge would be studied, she said.

Homendy said before a U.S. Senate committee last month that the investigation was focused on the ship’s electrical system generally.

Homendy said investigators were collecting information from the vessel’s engine room and circuit breakers, which she said “will help us tremendously.”

Marcel Muise, safety board investigator in charge, previously laid out a preliminary timeline assembled from the voyage data recorder comprising audio from the bridge and radio exchanges ahead of the collapse. He said experts would review the entire voyage data recording and develop a detailed transcript.

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