The timing and circumstances of this disaster are strange and the accident chain would have had to have been pretty unbelievable for this to play out the way it did.
A serious investigation is warranted.
Most accidents have a pretty unbelievable set of circumstances to happen. That’s why they don’t happen more often. It’s just like the lottery, astronomical odds still happen. You can look at the situation and think “there’s like a 1 in 100,000,000 chance of this happening” but then remember this bridge nearly 50 years old, and you can wonder how many ships have gone under it. Over a long enough stretch of time astronomical odds become eventualities.
I’m not so sure that 1.5 knots was not the average speed during some portion of the time period* of the collision.
*It might take a full second or more for the ship to come to a full halt, I’m guessing.
(Yeah, I know, double negatives...)