U.S. Defense Secretary Chuck Hagel has ordered a security review of U.S. military facilities worldwide following the mass killings at the Washington Navy Yard this week.
Hagel said warning signs about the shooter Aaron Alexis were missed, including his mental illness and legal problems.
"When you go back in hindsight there were some red flags, of course there were, and sure, should we have picked him up - why didn't we - how could we have - all those questions need to be answered," said Secretary Hagel.
Alexis, a former Navy reservist and military contractor, shot 12 people before he was shot dead by police Monday at the Navy facility.
His mother would not speak on camera, but read a statement to the families of the victims.
"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 very sorry that this has happened. My heart is broken," she said.
Monday's violence is the latest in a rash of deadly mass shootings in the United States. During an interview on the Spanish language television network Telemundo, U.S. President Barack Obama called on Congress to approve new gun control measures to keep guns out of the hands of the mentally ill.
"The fact that we do not have a firm enough background check system is something that makes us more vulnerable to these kinds of mass shootings. And I do get concerned that this becomes a ritual that we go through every three, four months, where we have these horrific mass shootings and yet we're not willing to take some basic actions that we know could make a difference," said Obama.
President Obama says the fact a firm enough background check system is not in place is something that makes the country more vulnerable to mass shootings like the one at the Washington Navy Yard.