Wrongful death and personal injury lawsuits were filed against United Airlines by or on behalf of those who were killed or injured in the September 11, 2001 terrorist attacks, when terrorists hijacked United Airlines Flight 175 and flew it into the World Trade Center. The lawsuits alleged that gross negligence by United and its security contractor, Huntleigh USA, allowed five terrorists to board the plane at Logan International Airport in Boston. The lawsuits sought to hold the defendants liable for airport security checkpoint failures, poor training, faulty cockpit doors, and failure by the airline industry to acknowledge and adequately respond to government warnings about terrorist threats. Although all settlements are confidential, a court document filed in 2009, when all but three lawsuits had been settled, stated that about $500 million had been paid out to resolve the claims.