tonight with a mystery buried inside a new clue left behind by the gunman at the washington navy yard. Tonight investigators studying these words. They were etched on a gun used by aaron alexis.... See More
tonight with a mystery buried inside a new clue left behind by the gunman at the washington navy yard. Tonight investigators studying these words. They were etched on a gun used by aaron alexis. Pierre thomas has the first take on what these riddles could mean. Reporter: This is a remington 870 pump action shotgun like the one used in the washington navy yard massacre a devastating close range weapon. Sources tell us that left an horrific scene of carnage in it's wake. Abc's has learned that aaron alexis carved the words better off this way" and "my e-l-f weapon" on the stock of the shotgun. Investigators are examining the meaning of the phrases including whether "better off this way" meant alexis knew he would die in his murderous rampage. As for elf authorities are not sure what it means though some investigators wonder if it refers to the tech phrase "extremely low frequency." Part of the curiosity about that phrase stems from the fact that alexis was a computer technician and was recently ranting to police about hearing voices and microwave machine sending vibrations to his body. That bizarre tale was told to police in rhode island who alerted the newport naval station police. But the navy confirmed today the information was never passed up the chain of command. And alexis kept his job as a civilian contractor with the navy, holding a secret clearance. There were a lot of red flags. Why they didn't get picked, those are all legitimate questions that we're going to be dealing with. Reporter: We heard today from a killer's crest fallen mother. She did not want her face shoun. Our son murdered 12 people. Aaron is 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 sorry that this happened. My heart is broken. Reporter: After hearing those voices alexis recently went to the virginia hospital complaining of insomnia. When asked of thought hurting anyone or himself he said no. That was that.
This transcript has been automatically generated and may not be 100% accurate.