Shooting in Chattanooga

Investigators combed through social media and backtracked through the life of the gunman whose rampage left four Marines dead, seeking clues Friday on how a popular engineering student drifted to apparent despair in a world he described as a “prison.”

While no clear motive has been determined for Thursday’s bloodshed in Chattanooga, the probe delved into possible ties to Islamist terrorist groups or whether the attacker — 24-year-old Mohammad Youssef Abdul­azeez — acted independently in a half-hour spree of violence that ended with his death by apparent police gunfire.

A day after a gunman opened fire at two military centers in Chattanooga, Tenn., and killed four Marines, authorities are trying to answer the big question: Why?

At a late-night press conference, authorities said they had yet to pin down a motive. But earlier, officials identified the shooter as 24-year-old Mohammod Youssuf Abdulazeez, who died during the course of the attack.