He had traveled to Yemen

The killer of the volunteer recruiter had some history:

ABC News has learned that Muhammad had been under investigation by the FBI's Joint Terrorist Task Force since his return from Yemen.

The investigation was in its preliminary stages, authorities said, and was based on the suspect's travel to Yemen and his arrest there for using a Somali passport.

Yemen. The birthplace of Osama bin Laden. The site of the USS Cole bombing. They had good reason to watch him. Too bad the investigation was still preliminary.