MAZAR-I-SHARIF, Afghanistan — Mohammad Nasim knew he would not be searched at the gates of the International Committee of the Red Cross orthopedic center because he was a polio patient who had been going there for 19 years.

“They did not imagine I could do something like this,” he said.

So he made no special effort to hide the Russian-made Tokarev semiautomatic pistol — just tucked it under the shirt of his shalwar kameez and rolled himself down the main corridor in his wheelchair.

Lorena Enebral Perez, a physiotherapist from Spain who had started working there a few days before, stepped into the corridor, headed for the 10 a.m. tea break. She had just finished a session with children; pediatric physiotherapy was her passion.

Mr. Nasim went right up to her in his chair. He says he did not know who she was, but guessed — incorrectly — that she was the center’s director. He pulled the pistol out and, without a word, shot her in the heart at nearly point blank range.