The carnage in Jos was devastating. There was an explosion at the Yantaya mosque, and worshipers began to flee. Umar Farouk Musa, a spokesman for J.N.I., or Jama’atu Nasril Islam, the country’s main Muslim organization, said that after the blast, “people emerged from nowhere, five of them, holding sophisticated arms, shooting sporadically into the crowd.”

Mr. Musa said he witnessed the mosque attack. “We saw these people emerging from nowhere, covered with blankets,” he said in a telephone interview from Jos, referring to the gunmen. “Before we knew it, they disappeared into thin air. There were victims beyond what we could count.”

Mr. Musa said he had seen “more than 100 dead bodies,” though Nigerian media put the total death toll for the two attacks in Jos at half that figure.

The other attack, at a nearby site opposite the university in Jos, happened a few minutes earlier, when a suicide bomber walked into a restaurant and detonated his charge. Mr. Musa and colleagues arrived on the scene shortly afterward and “counted 15 dead bodies,” he said.