I didn't read all the details of the sad deaths while cleaning...

I didn't read all the details of the sad deaths while cleaning the tank this week. I know they were using a petrol pump but surely didn't have it in the tank?. Sad as was one brother collapsed in tank and other seemingly went in to help and then wife and all 3 overcome and dead. Pump may have been up top of concrete in ground tank? but carbon monoxide is heavier than air and went in via hatch? or was it fumes from the sludge?

Have cleaned a few tanks and silos and we always had a rope and harness but not sure I'd do it same ways again