Why are 9/11 victim's families suing airline MANUFAUTERER'S??

Look, I can understand a lawsuit against the airport security firm, and the airports themselves, but why sue(I'm uming Boeing or Airbus) the plane's manufacturer?? Did they ever release a statement saying their airplanes were hi-jack proof?? This seems to me like the attorney's involved are suing any possible firm's involvment in the event to try to maximize their pay-out...Is anyone else irritated by this??