At Fort Hood, which sprawls for 340 square miles over the Texas prairie, Specialist Lopez was being treated for behavioral and mental health issues. To enter the base, he would have undergone no security screening beyond showing his identification and would have passed through no metal detectors.

Military personnel who are not police officers are not allowed to carry privately owned weapons on Army bases. Soldiers on post must register their firearms, which Army officials said Specialist Lopez failed to do with the handgun he used in the attack. Fort Hood’s rules rely in large part on the honor system, and require all personnel bringing a privately owned firearm onto the base in a vehicle to declare that they are doing so and state why.

“Fort Hood is a big installation,” the base’s commanding general, Lt. Gen. Mark A. Milley, told reporters on Thursday. “We’ve got a population well over 100,000 here. It would not be realistic to do a pat-down search on every single soldier and employee on Fort Hood for a weapon on a daily basis.”