Cathleen Alexis said Wednesday in New York City that she does not know why her son, Aaron, did what he did and she will never be able to ask him.
The mother read a statement at her home in Brooklyn. She concluded, "My heart is broken."
Thirty-four-year-old Aaron Alexis opened fire at the Navy Yard on Monday before he was killed in a shootout with police.
Authorities say he had been suffering a host of serious mental problems including paranoia and a sleep disorder.
Meanwhile, operations at the Washington Navy Yard still haven't returned to normal, and the facility is once again open only to mission-essential personnel today.
The Navy says the yard remains an active crime scene.
Meanwhile, more details are emerging of the shooting rampage and gun battles that lasted more than a half-hour on Monday. The FBI says the average mass shooting is over within minutes, and often ends once police arrive.
The rampage by a defense contractor is raising questions about the adequacy of the background checks done on contract employees who hold security clearances.
Navy and White House security reviews have begun as a picture emerges of the gunman as an agitated and erratic figure whose behavior and mental state had repeatedly come to authorities' attention but didn't seem to affect his security clearance.