At least 12 people were killed and 31 wounded at the military facility after up to three gunman opened fire, military officials said.
Obama offered his "thoughts and prayers" to the families of the victims.
"These are men and women who have made the selfless and courageous decision to risk, and at times give, their lives to protect the rest of us on a daily basis," he said.
"It's difficult enough when we lose these brave Americans in battles overseas. It is horrifying that they should come under fire at an army base on American soil."
Obama said he had spoken to both Defense Secretary Robert Gates and Admiral Michael Mullen, chairman of the Joint Chiefs of Staff, and would receive "a constant stream of updates."
Information about the perpetrators of the attack and their motives remained unclear but Obama pledged: "We will make sure that we get answers to every single question about this horrible incident."
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Image: US Army soldiers train.
Obama offered his "thoughts and prayers" to the families of the victims.
"These are men and women who have made the selfless and courageous decision to risk, and at times give, their lives to protect the rest of us on a daily basis," he said.
"It's difficult enough when we lose these brave Americans in battles overseas. It is horrifying that they should come under fire at an army base on American soil."
Obama said he had spoken to both Defense Secretary Robert Gates and Admiral Michael Mullen, chairman of the Joint Chiefs of Staff, and would receive "a constant stream of updates."
Information about the perpetrators of the attack and their motives remained unclear but Obama pledged: "We will make sure that we get answers to every single question about this horrible incident."
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Image: US Army soldiers train.