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 March 26 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.

The bridge collapsed within seconds after the cargo ship Dali lost power and struck one of its supporting columns. 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.

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