It will be interesting if there is any live footage, of the failure, that would tell you a lot of clues right away. I was a little unfair, we know little, and I shouldn't blame the engineer.....

Human failure in construction accidents is, by far, the most likely cause, when these sort of things happen.... And, the students who witnessed it and posted immediately afterward on YouTube noted that there were workers on the bridge when it collapsed.

That one you mentioned is called.... "Now did the foreman say to take this support out first, or that one.....?"

Materials failure, or short curs are another place to screw up..... That one goes like this....."You mean you really wanted me to use the really expensive bolts in the specs....but I got these in the same size for a third of that price.......?"

Although, engineers can screw up too, for various reasons. I once watched the engineers, in department A, write a spec to buy LRT cars with Japanese profile flanges, for LRT track that had been built by the engineers in department B, with an American rail profile, and then proceed to have "operations and maintenance" "eat the cost" to peel brand new wheel sets, so the cars would not pick the switches, and derail.....

And, I got to witness the occasion that the civil engineers at a certain State DOT built a segment of brand spanking new freeway, that actually started breaking up before it was even open, because we'd done a cut, without checking for the level of the ground water, in a known flood plain. The design profile had cut down to within ten feet of the water table in some spots, and then poured hundreds of tons of concrete on top of a sub surface that was completely saturated..... Oops....

So I have seem plenty of projects where there are more than ample of opportunities for error leading to dangerous and costly mistakes, by all the varied participants and professions.

This is an unfortunate tragedy for the victims, regardless of ultimate cause. But it is going to be an interesting investigation.....