BALTIMORE (AP) — The electrical blackouts experienced by the container ship Dali before it left Baltimore’s port were “mechanically distinct from” those that resulted in the deadly collapse of the Francis Scott Key Bridge hours later, according to congressional testimony Wednesday.

“Two were related to routine maintenance in port. Two were unexpected tripping of circuit breakers on the accident voyage,” National Transportation Safety Board Chair Jennifer Homendy testified.

The Dali was headed to Sri Lanka, laden with shipping containers and enough supplies for a monthlong voyage. Shortly after leaving the Port of Baltimore early on March 26, the ship lost power and propulsion and crashed into one of the bridge’s supporting columns, killing six construction workers.

Homendy’s remarks came the day after the safety board released its preliminary report into the bridge collapse. Their full investigation could take a year or more.

The ship’s first power outage occurred after a crew member mistakenly closed an exhaust damper while conducting maintenance in port, causing one of its diesel engines to stall, according to the report. A backup generator automatically came on and continued to run for a short period — until insufficient fuel pressure caused it to kick off again, resulting in a second blackout.

While recovering from those power outages, crew members made changes to the ship’s electrical configuration, switching to a different transformer and set of breakers, according to safety investigators.

“Switching breakers is not unusual but may have affected operations the very next day on the accident voyage,” Homendy testified Wednesday morning before the U.S. House Committee on Transportation and Infrastructure.

She said the board is still gathering more information about what exactly caused the various power outages. The FBI has also launched a criminal investigation into the circumstances leading up to the collapse.

When the breakers tripped as the Dali approached the bridge, Homendy said the ship’s emergency generator kicked on. That generator can power the ship’s lights, radio and other operations, but it can’t restore propulsion.

“Without the propeller turning, the rudder was less effective,” Homendy said. “They were essentially drifting.”

While there is redundancy built into the ship’s systems, she said it’s not unlike other vessels in terms of the functions of its emergency generator and other factors. She said investigators are working closely with Hyundai, the manufacturer of the Dali’s electrical system, to pinpoint what went wrong after it left the Port of Baltimore.

“We are still on scene and evaluating everything about this accident,” Homendy said.

She also reiterated another finding from the report, which said testing of the ship’s fuel revealed no concerns about its quality.

The safety 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.

The preliminary report details the chaotic moments prior to the bridge collapse while crew members scrambled to address a series of electrical failures that came in quick succession as disaster loomed.

At 1:25 a.m. on March 26, when the Dali was a little over half a mile away from the bridge, electrical breakers that fed most of the ship’s equipment and lighting unexpectedly tripped, causing a power loss. The main propulsion diesel engine automatically shut down after its cooling pumps lost power, and the ship lost steering.

Crew members were able to momentarily restore electricity by manually closing the tripped breakers, the report says.

The ship was less than a quarter of a mile from the bridge when it experienced a second power blackout because of more tripped breakers. The crew again restored power, but it was too late to avoid striking the bridge.

A last-minute mayday call from the ship allowed police to stop traffic, but they didn’t have enough time to warn a team of construction workers who were filling potholes on the bridge. One man was rescued from the water. A road maintenance inspector also survived by running to safety in the moments before the bridge fell.

The last of the six victims’ bodies was recovered from the underwater wreckage last week.

On Monday, crews conducted a controlled demolition to break down the largest remaining span of the collapsed bridge, which landed draped across the Dali’s bow. The ship is expected to be refloated and guided back to the Port of Baltimore early next week, officials said Wednesday.