US Defense Secretary Chuck Hagel has acknowledged security gaps after a deadly shooting at a military base in Texas but appealed for patience to allow investigators to "do their work."
Hagel said on Thursday it was too soon to draw any conclusions as a litany of questions remained unanswered 24 hours after a soldier killed three people at Fort Hood before turning the gun on himself.
"Let the investigators do their work. We don't know all the facts," Hagel told a news conference in Honolulu, where he hosted a meeting of ASEAN defence ministers.
"What motivated this person to do this? Where was the gap? Why did we have a gap? Why did it happen?" Hagel said.
"We are going to find out."
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The Pentagon chief said the department had recently introduced new measures to tighten security after a shooting spree at the Washington Navy Yard in September that claimed 12 lives.
But he said: "Obviously we have a gap."
He vowed "to do everything possible to implement the kinds reforms that fill those gaps and assure the security of the men and women who work for our armed forces and assure their families".
Hagel declined to be drawn on whether the incidents reflected a systemic problem at military bases or a wider pattern of gun violence across American society.
He said while the criminal probe continues, "we will all stay focused on the victims and their families and the Fort Hood community, who yet again are experiencing a terrible tragedy and much grief".
The sprawling US Army base in Texas suffered another shooting spree in 2009, when an officer turned Islamist extremist killed 13 people.
