From a New York Times story on mental health:
On Monday, Mr. Alexis entered the Navy Yard, a secure military facility near the Capitol, killing at least 12 people before he was fatally shot by the authorities. The police say Mr. Alexis, 34, acted alone. On Tuesday, the authorities said it had taken only two minutes for officers armed with AR-15s to arrive at the navy yard once they received the first of many 911 emergency calls.
The authorities said that while they had originally believed that Mr. Alexis had been armed with an AR-15 that they now believe he entered the navy yard with only a shotgun and had obtained a pistol once inside.
The rest here.