Federal investigators looking into what caused a ship to crash into a vital Baltimore bridge in March are honing in on the vessel’s electrical system, U.S. National Transportation Safety Board Chair Jennifer Homendy said on April 10.

The safety board is examining data on the ship’s electronic power system and circuit breakers, she said in testimony before a Senate panel.

“That is where our focus is right now,” she told the Senate Committee on Commerce, Science and Transportation, noting that it’s still very early in the process. “We have a lot of work to do still on this investigation.”

The March 26 bridge collapse killed six and shut down the Port of Baltimore. The Singapore-flagged cargo ship slammed into the bridge overnight, causing billions in damages. The vessel couldn’t maintain its desired heading after experiencing a loss of propulsion, but the reason for the failure remains under investigation by the NTSB. Investigators are still on the scene, with the board’s preliminary report expected to be released the first week of May.

Video of the stricken ship showed its lights flashing on and off in the moments before collision. In response to a question from Senator Ted Cruz, the panel’s top Republican, Homendy said she believed that was related to the incident. 

In addition, fuel from the ship is being tested to see if it was a factor, Homendy told reporters following the hearing. Tainted shipping fuel has resulted in serious and sometimes complete engine failure that has left ships floating helplessly in the open sea, according to a 2018 report by the Washington-based think tank Atlantic Council.

The NTSB has investigated many bridge strikes in the past and in the early 1980s issued recommendations to the U.S. Coast Guard to evaluate protection of bridge piers, among other things, Homendy said. At the time, the Coast Guard said it didn’t have the authority to do so, she said.

“There is still action that needs to occur,” Homendy said. In the decades since, waterway transportation has changed, including the use of larger container ships. “Are these bridges protected for the type of traffic that is going through now?”