The Dali container ship experienced a near-perfect storm of calamities before it struck Baltimore’s Francis Scott Key Bridge in March, killing six people. But Tuesday’s preliminary report by federal safety investigators leaves many questions unanswered.

For example, the National Transportation Safety Board described four power failures that beset the 984-foot (300-meter) ship. Reasons for three of them are yet to be explained, including two that occurred right before the crash.

Here’s what we know and don’t know about the problems on the ship before the disaster:

HOW MANY TIMES DID THE SHIP LOSE POWER?

The Dali suffered four blackouts in roughly 10 hours: two the day before the crash and two in the minutes before.

The first came after a crew member mistakenly closed an exhaust damper during maintenance, causing one of the ship’s diesel engines to stall, according to the safety report. A backup generator came on, but insufficient fuel pressure soon caused that generator to kick off, resulting in a second blackout. It’s unclear why the fuel pressure dropped.