The one question we all desperately want answered may have gone to the grave with Aaron Alexis: Why?
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," says Valerie Parlave of the FBI. "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.
What we know: Senate Majority Leader Harry Reid may have said it best Tuesday when he told colleagues that "there is no explanation for the violence." So far, investigators haven't offered any clue as to what motivated Alexis to kill.
Friends say he had been frustrated over pay and benefits issues from a previous contracting job, and federal law enforcement sources said he may have contacted Veterans Affairs hospitals for possible treatment of psychological issues. But so far, investigators haven't said if either of those issues contributed to the attack.
What we don't know: What he might have left behind in his Washington hotel room, either in writing or on any computers FBI agents may have found while searching the room.
Terrorism hasn't been ruled out but seems unlikely, according to Washington Mayor Vincent Gray.
What we know: Throughout the day, authorities said they were looking for a second man. But by nightfall, they said they were "confident" that Alexis was the lone gunman. "We have exhausted all means to eliminate that possible last suspect," said Metropolitan Police Chief Cathy Lanier. "So we do now feel comfortable that we have the single and sole person responsible for the loss of life inside of the base."
On Tuesday, Parlave, the assistant director in charge of the FBI's Washington field office, underscored that, telling reporters it's believed that Alexis acted alone.
What we don't know: While authorities have stated they don't think anyone else besides Alexis opened fire at the Navy Yard, that doesn't mean others didn't help him or know about the plot.
Alexis arrived in Washington on or around August 25, staying in hotels in the capital ever since, said Parlave. Shawn Henry, a former executive assistant director of the FBI, said he believed authorities will be talking to people Alexis has known for years -- including those he went to school with and served in the Navy with -- as well as looking at things like bank and cell phone records.
What we know: We now have all the names of 12 victims. They range in age from 46 to 73. Another eight people are recovering from wounds inflicted in the shooting, including three at MedStar Washington Hospital Center.
Washington Mayor Vincent Gray said many of the victims gunned down in the Navy Yard's Building 197 were eating breakfast when Alexis shot at them from above.
What we don't know: We have yet to find out more about many of the victims, such as what they did at the Navy Yard, where they were at the time of the shooting, etc.
What we know: Washington Police Chief Cathy Lanier said Tuesday that Alexis had "multiple contacts" with officers from several agencies during the shooting spree. The ordeal ended when Washington police Officer Scott Williams fatally shot Alexis, 34, Mayor Vincent Gray told CNN.
What we don't know: Exactly how Alexis died, beyond the fact it happened after 30 minutes of gunfire, according to Gray.
What we know: He left New York after apparently helping rescue efforts at the site of the World Trade Center after the September 11, 2001, terror attacks, according according to a source with direct knowledge of the investigation. His father told Seattle police after a 2004 arrest that Alexis was suffering from post-traumatic stress disorder related to that experience.