Why did he park at the Washington Navy Yard on Monday, walk into Building 197, perch himself on an overlook above the atrium and open fire? The bullets that rained down killed 12 people and wounded eight others.
But that’s not the only missing puzzle piece. Investigators are painstakingly trying to piece together the motive, the means and the method.
“No piece of information is too small,” said Valerie Parlave of the FBI said Monday night. “We are looking to learn everything we can about his recent movements, his contacts and associates.”
For now, here’s what we know and what we don’t know.