Air's answer is pretty thorough already, and the liability issue is truly complex. In my liability training at work, we've been told of situations where engineers from every firm with any association to the machinery or structure in question are named in suits, not necessarily because they're all at fault, but (1) so that their fault can be determined by a judge and (2) because in a civil case, settling may be cheaper than paying for a lawyer to try and get the company/engineers dismissed from the suit.
What I want to do is point out a good example of a catastrophic failure, the level of research that can go into finding out what happened, and the consequences for the firms and engineers involved. One of my professors favorite case studies was the Kansas City Hyatt Regency walkway collapse.
In short, two walkways, at the fourth and second floor levels and directly above/below one another, collapsed during an event the hotel hosted, killing 114 people and injuring. It was the largest loss of life in a structural collapse in the US from the time that it happened in 1981 until the World Trade Center collapsed on Sept 11th.
There was a large investigation by the National Bureau of Standards, a link to which can be found here. That PDF is 370 pages long, including 95 pages of appendices.
As a result of the investigation, the engineering firm and the engineers who signed off on the final drawings lost their licenses to practice, though it appears they avoided any criminal charges and that most of the civil damages were paid by the owners of the hotel.
Obviously, all cases are different, so while they criminal charges were dismissed here, it's possible they wouldn't be in another case. I think the important thing to take away here is that a lot of the work engineers do can be dangerous if not done properly, and being involved on a project that ultimately injures someway means you are extremely likely to have to answer for your design choices at some point, even if they are sound and you have done nothing wrong.