Navy Yard Shooter Carved 'BETTER OFF THIS WAY' Into Shotgun Aaron Alexis also carved MY E-L-F WEAPON into stock of gun.

Sept. 18, 2013 -- Navy Yard gunman Aaron Alexis carved the words "BETTER OFF THIS WAY" into the gun he used in his shooting rampage in Washington, .D.C, ABC News has learned.

He also carved the phrase "MY E-L-F WEAPON" onto the stock of the shotgun, sources said.

Investigators said they are examining the meaning of the phrases, including whether "better off this way" meant Alexis knew he would die in his killing spree.

Authorities are unsure about the significance of ELF but said it can mean "extremely low frequency" in some military and tech circles. There is some speculation about the term because of Alexis' rants about hearing voices and "vibrations sent to his body" by a " microwave machine," according to law enforcement sources.

Alexis told police in Newport, R.I., in August that he heard voices that were trying to send vibrations into his body at a hotel in Newport.

Police are still investigating a motive for the Monday morning shooting, during which Alexis killed 12 employees of the Navy Yard and injured others before he was killed by police gunfire.

Earlier today, the mother of Aaron Alexis, the gunman who killed 12 people at the Washington Navy Yard, said today that her "heart is broken," but she is "glad" that he can no longer do harm to anyone.

Alexis' mother, Cathleen Alexis, read a brief statement to reporters in response to her son's murderous rampage Monday at the Navy Yard, saying she had no idea why he would do such a thing.

"Our son Aaron Alexis has murdered 12 people and wounded several others," she said in the statement. "His actions have had a profound and everlasting effect on the families of the victims. I don't know why he did what he did and I'll never be able to ask him why."

"Aaron is now in a place where he can no longer do harm to anyone and for that I am glad. To the families of the victims I am so, so very sorry that this happened," she said. "My heart is broken."

Alexis' family has spoken only briefly about their son since the shooting on Monday morning, in which Alexis entered the Navy base where he was set to begin an IT contracting job and opened fire on employees. Twelve individuals were killed before law enforcement officers were able to shoot and kill Alexis.

On Tuesday, Alexis's stepfather told the Daily Mail that Alexis was walking out of the World Trade Center subway stop when the first tower collapsed, and that he stayed to help pull bodies from the rubble.

"Aaron said he was on the escalator coming up from the subway station when he saw the building come down. It was right in front of him, and he said he saw it collapse," Frank Calderon, Alexis's stepfather, told the Daily Mail. "That day was traumatic for everyone involved. He did not talk much about it, but he said he helped out. He was a strong young man, so he would have done what he could."

On Monday, Alexis' brother-in-law came out of the family's home in Brooklyn, N.Y., and told reporters that Alexis did not come home often. Alexis' sister had not seen her brother in five years, her husband said.

In the hours after the shooting, police reports from around the country showed Alexis' encounters with police over the past decade exhibited a pattern of angry outbursts and disturbing behavior.

He told friends and police that he suffered from post-traumatic stress disorder because of his work at the 9/11 World Trade Center attack, a claim that New York authorities said they had no record of. Alexis's father repeated the claim to Seattle police after Alexis was arrested in 2004 for firing a gun at a man's car tires. His father said Alexis suffered from PTSD because of 9/11.

A police report in August from Rhode Island stated that Alexis had called police to say that three men were following him and they were sending vibrations through his body. Alexis also said he was hearing voices.