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Tuesday, two senior military officials told CNN that security cameras monitoring the perimeter of the outpost showed the sergeant leaving the outpost alone and returning alone. Investigators are looking into whether alcohol may have been a factor in the attack, though toxicology tests on the suspect were not complete, the officials said.

Alcohol was found on the outpost in the area where the suspect was housed, but it was not clear whether it belonged to the sergeant, one of the officials said.

Speaking at the White House, Obama said he has told Karzai that the United States "takes this as seriously as if it was our own citizens and our own children who were murdered."

"The killing of innocent civilians is outrageous and it's unacceptable. It's not who we are as a country and it does not represent our military," he said. A U.S. military investigation "will follow the facts wherever they lead us, and we will make sure that anybody who was involved is held fully accountable with the full force of the law," he added.

Allied combat troops are scheduled to leave Afghanistan by 2014, and Obama said the number will be reduced by more than 30,000 by summer's end, matching the number of Americans he sent in after taking office.

"There's no question that we face a difficult challenge in Afghanistan, but I'm confident that we can continue the work of meeting our objectives, protecting our country and responsibly bringing this war to a close," he said.

Mark Jacobson, a former official with the NATO command in Kabul, said the reaction to Sunday's massacre may not force a shift in allied strategy, but could cause "an increase to the pace of transition." But the demonstrations have been muted compared with the deadly riots that followed the disclosure that American troops had burned copies of the Quran -- mistakenly, their commanders said.

"Not to be callous in any way, but the Afghans are in some ways used to the civilian casualty incidents and tend to respond to it in a more reserved manner than something that seems to be a direct and personal affront to their religion and culture," Jacobson said.

In other incidents adding to the strain on U.S.-Afghan relations, U.S. commanders condemned a video of a squad of Marines urinating on bodies in January, and several soldiers -- from Joint Base Lewis-McChord in Washington state, like the suspect in Sunday's shooting -- were charged with taking part in a rogue "kill squad."

As he set out Monday for a trip to the region, Panetta said the United States and its NATO allies "seem to get tested almost every other day." But, he added, "It is important that, all of us, United States, Afghanistan, the (NATO-led) forces all stick to the strategy that we've laid out."

"War is hell," he said. "These kind of events and incidents are going to take place. They've taken place in any war. They're terrible events. This is not the first of those events, and they probably won't be the last."

The suspect in the attack served three tours of duty in Iraq before being deployed to Afghanistan, Allen said.

During the suspect's last deployment, in 2010, he was riding in a vehicle that rolled over in a wreck, according to a senior Defense Department official, who also spoke on condition of anonymity. After the wreck, the sergeant was diagnosed with traumatic brain injury but was treated and then found fit for duty, the official said.

CNN's Chris Lawrence, Barbara Starr, Jethro Mullen and journalist Ruhullah Khapalwak contributed to this report.

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