1. Investigators were searching for clues to why a former Marine armed with a .45-caliber handgun opened fire at a crowded California dance hall on Wednesday night, killing 12 people.

He was identified as Ian David Long, 28. He died after being confronted by police officers, including one who became his victim, a 29-year veteran of the local sheriff’s office, Sgt. Ron Helus. Above, an investigator interviewing a witness.

Witnesses described a chaotic scene at the club, which was filled with hundreds of people, many of them college students. Patrons rushed toward the exits and broke windows to escape.