Associated Press
    Aaron Alexis.

    By Devlin Barrett and Jacob Gershman

    There’s no shortage of questions surrounding Monday’s Navy Yard shooting spree. Here are four mysteries that authorities are grappling with in the aftermath.

    Why was the alleged gunman able to get a gun into Building 197? Authorities say the alleged suspect, Aaron Alexis, shot a security officer at an entrance into Building 197, which houses the Naval Sea Systems Command. At most government buildings, the security at the building entrance is supposed to prevent a shooter from being able to go inside, even if front-door security is attacked. Video footage though indicated that he didn’t shoot his way into the building, a defense official told WSJ. The official said the guard was later killed in the shooting.

    Why wasn’t Alexis dishonorably discharged? Officials say there were multiple instances of problematic behavior that got Mr. Alexis pushed out of the Navy, including a 2010 arrest for firing a gun at his Fort Worth apartment. Other conduct offenses involved insubordination, disorderly conduct and unauthorized absences from work. “There is no question there was a pattern of misconduct,” a defense official told WSJ. Mr. Alexis, though, left the military voluntarily after he was threatened with a dishonorable discharge. By quitting, he apparently was able to keep his security clearance. Military officials say they lacked enough information to outright dismiss him.

    Who conducted the background checks and how rigorous were the reviews? The FBI said late Monday that Mr. Alexis allegedly gained access to the Navy Yard with a valid pass obtained as part of his contractor work. He held a secret security clearance through his employer, The Experts. The company’s CEO, Thomas Hoshko, told WSJ that he had no awareness of Mr. Alexis’s troubled past — which included two earlier brushes with the law involving guns — and said the military was in charge of the employee screening process. The suspect recently underwent a new security background review, according to Mr. Hoshko.

    What was the motive? Authorities said they had yet to determine a motive, but had found nothing to suggest a connection to terrorism. Officials say Mr. Alexis had a history of mental problems and sought treatment for paranoia. So we may get clues, but we may still never know for sure what drove the alleged shooter to commit mass murder.

    – Dion Nissenbaum contributed to this post.