Navy Yard gunman's sister on opportunities missed

Aaron Alexis shot and killed 12 people on September 16, 2013, at the Washington Navy Yard, in southeast Washington D.C. The 34-year-old used a valid temporary ID to get into Building 197, and was eventually shot dead himself by police.

FBI agents later found a note from Alexis claiming that "ultra-low frequency" waves had been attacking him for months.

In a "CBS This Morning" exclusive interview, his sister, Naomi Alexis, shed light on the mental health problems that consumed her brother, reports CBS News correspondent Vicente Arenas.

The man she describes when talking about her brother is one with a dark and dangerous past -- a past so haunting that Naomi and her sister weren't surprised when they found out Aaron was the shooter.

"About 2:00 in the afternoon someone called her cell phone and said, 'Do you -- would you like to make a statement about your brother?'" she recalled.

When she searched the internet, the truth became apparent.

"I Googled his name and saw that it was him," she said.

His two sisters, including Naomi, heard about the shooting on the radio and had a feeling their brother was involved.

"There was a report of an assailant and -- he was a six-foot African American male," Naomi said. "And my sister and I were at home and she said, 'it would be crazy if that was Aaron.'"

Naomi believes Aaron suffered from severe mental problems that started when he was an adolescent. She said he would violently beat her when she was just 12 years old.

"It was unprovoked a lot of the time, and a lot of time it was just over silly things," she said. "Like, I spoke while he was on the phone so he would, you know, punch me in the face."

She said he hit her hard.

"Oh yeah, I mean I was swelling," she said.

The pain never went away and even in death she continues to run from him. After the shooting, she moved from New York and has asked us not to mention where she lives now. This is the first time she's spoken at length about the shooting and her brother.

"Oh, I don't want to cast blame. I mean, there are so many -- so many instances where this would've been addressed," she said.

On the barrel of the shotgun that was recovered, the words, "end the torment," were written.

"I think my brother was suffering," she said. "I think he was scared; he called the police and said, 'I'm hearing -- they're making all this noise.'"

His statements to police didn't surprise her. When he was a teenager, she says, he'd lock himself in his room blasting music for hours. Then came stories of torturing pets. In 2004, Aaron was arrested in Washington State for shooting out the tires of another man's vehicles. According to the Navy, Alexis was cited for misconduct at least eight times.

Over the years, Naomi told her family he needed to see a doctor.

"I would have violent, like, tearful outbursts and try to get my mom to be like, 'This is not right, there's something you can... this is not supposed to be this way,'" she said.

But Aaron's actions were something the family just didn't discuss.

"I think absolutely there was a sense of shame or maybe helplessness," she said. "They didn't know how to address the situation without, you know, labeling my brother mentally ill."

Naomi is telling her story hoping families who have similar problems will call the National Alliance on Mental Illness to get help for their loved ones. She's also hoping to write a book about her experience, hoping her family's story will help others.