Details Surface of Navy Yard Shooter Aaron Alexis

While the nation mourns the victims of the Navy Yard shooting in Washington D.C., more information is coming to light on the shooter, Aaron Alexis. INSIDE EDITION reports.

The star-spangled banner echoed across the stadium just two blocks from the Washington Navy Yard, scene of the slaughter. There was a moment of silence in memory of the 12 who were gunned down.

The task of singing the national anthem on such an emotionally charged day fell to a TSA agent, Tabatha Messenberg, who said, "It was definitely an honor."

As it has after other tragedies, baseball is helping heal wounds and bring the nation together.

Meanwhile, a terrifying and puzzling portrait of Aaron Alexis is now emerging, with some recalling him as a sweet-natured guy who would never harm a fly, while others recall an angry man with a hair-trigger temper.

The killer's brother-in-law, Anthony Little spoke to INSIDE EDITION outside the family home in Brooklyn, New York.

"It's devastating. It's the day after and it's really starting to sink in more. It's really hurtful. I'm super shocked," said Little.

Aaron Alexis was 34 years old. He was being treated for serious mental illness and was also hearing voices. The last days of his life were spent at a Residence Inn hotel in Washington.

In the days leading up to the massacre, Alexis stayed in a modest room at the Residence Inn, not far from the Navy Yard. The room is designed for people who live on the road for days at a time, like government contractors. It comes complete with a little kitchenette and a desk. The going rate is about $226 a night.

Alexis was 6-foot-one and weighed 190 pounds, according to an FBI poster which has been updated to list him as deceased.

He served in the Navy in Fort Worth, Texas, and was discharged after a pattern of misconduct. He found work as a waiter at a Thai restaurant in Fort Worth, and was a practicing Buddhist.

Michael Ritrovato says Alexis was a good drinking pal. 

"We were good buddies. I would call us friends," said Ritrovato.

He says Alexis had only one disturbing habit.

"He spent a lot of time with violent video games. Bloody, violent, shooting, blow your head off type things," said Ritrovato.

According to his father, Aaron Alexis had "anger management problems" caused by the post-traumatic stress disorder he suffered after working as a rescuer after the terrorist attacks of September 11, 2001.

A troubled life, a violent end.

Among the 12 killed by Alexis were 51-year-old Mary DeLorenzo Knight , 46-year-old Kenneth Proctor and John Roger Johnson, the oldest of the victims at age 73.

Why did Alexis snap and go on his murderous rampage? It's a mystery that he has taken to his grave.