WASHINGTON - The mother of the gunman who shot dead 12 people at a US navy base this week said Wednesday she doesn't know why Aaron Alexis opened fire, but she is glad he can no longer hurt anyone else.

Cathleen Alexis read a brief statement inside her New York home, her voice shaking. She did not want to appear on camera and did not take questions.

"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,'' Cathleen Alexis said.

"To the families of the victims, I am so, so very sorry that this has happened. My heart is broken.''

Her son was killed in a shootout that ended a rampage that lasted more than half an hour. His motive was not known. Law enforcement officials and others have said Aaron Alexis, 34, heard voices and believed he was being followed.

He recently told police in Rhode Island he heard voices harassing him, wanting to harm him. He couldn't sleep. He believed people were following him, using a microwave machine to send vibrations to his body.

He was a naval defense contractor, and on Aug. 7, police alerted officials at the Newport Naval Station about his call. But officers didn't hear from him again.

By Aug. 25, Alexis had arrived in the Washington area, continuing his work as an information technology employee for a defense-related computer company.

He suffered from serious mental problems and was undergoing treatment from the federal Department of Veterans Affairs, according to the law enforcement officials. But Alexis wasn't stripped of his security clearance, and he kept working.

On Saturday, he visited a gun range in Virginia, not far from the nation's capital. He rented a rifle, bought bullets and took target practice, then bought a shotgun and 24 shells, according to the store's attorney.

On Monday, Alexis entered the sprawling Washington Navy Yard, which is protected by armed guards and metal detectors where employees must show IDs to get past doors and gates. He had his pass for base access _ and the shotgun, a Remington 870 Express.

The shotgun was brought into the building disassembled and pieced together by Alexis once inside, according to a law enforcement official and a senior defense official who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly.

He opened fire around 8:15 a.m., shooting down from a fourth-floor overlook and third-floor hallway into a glass-walled cafeteria. Alexis also took a handgun from an officer. He fired at police and workers in gun battles that lasted more than half an hour.

The FBI says the average mass shooting is over within minutes and often ends once police arrive. Alexis was killed in a shootout.

The Navy said the military installation would reopen for business for Mission Essential personnel only Wednesday. In a posting on its Facebook page, the Navy said the yard remains an active crime scene.

Meanwhile, Defense Secretary Chuck Hagel ordered the Pentagon to review the physical security of all US defense facilities worldwide and the security clearances that allow access to them.

"Where there are gaps, we will close them,'' he said.