Actually, when screening was first instituted, it was *only* at the gate. That avoided many potential 4th Amendment problems (because there is no justification to require people who are not traveling and do not have access to the aircraft (i.e. - most airport employees, visitors, etc.) to go through search). Then they decided it would be more convenient and cost-effective to have centralized screening. I think they made the change at LAX during the summer of 1971.