KABUL, Afghanistan (CNN) -- Afghan protesters angered by the weekend killings of 16 civilians blocked a major highway Tuesday as President Barack Obama said U.S. officials were "heartbroken" by the deaths.
A U.S. Army sergeant is accused of killing the Afghan villagers in what top commanders say was a rampage Sunday morning near Kandahar. A protest in Jalalabad, near the border with Pakistan, drew hundreds of people Tuesday, with demonstrators blocking the highway to Kabul, provincial government spokesman Ahmad Zaii Abdulzai said.
"We are in the process of trying to reopen," Abdulzai told CNN.
Afghan President Hamid Karzai issued a fresh denunciation of the attack Tuesday. And in Washington, Obama told reporters that "the United States takes this as seriously as if it was our own citizens and our own children who were murdered."
"We are heartbroken at the loss of innocent life," he said. "The killing of innocent civilians is outrageous, and it is unacceptable. It is not who we are as a country and it does not represent our military."
Obama 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."
The pre-dawn rampage left nine children, three women and four men dead in two villages near a combat outpost in Kandahar province, Afghan authorities said. The still-unidentified soldier blamed for the attack turned himself in after the killings and could face the death penalty, Defense Secretary Leon Panetta said.
The bloodshed is a fresh strain between Washington and Kabul, and Karzai said Tuesday that the killings "caused great pain for the people of Afghanistan." An Afghan official said he heard gunshots and grenades being launched Tuesday while a delegation of top officials visited one of the villages where the shootings took place.
"While we were in the village of Alokozai for a funeral, praying for a martyr killed in the massacre, we heard close-range, small-arms fire, followed by two rocket-propelled grenades," said Haji Agha Lali, a member of Kandahar's provincial council.
"According to my information, two to three Afghan security forces have been injured," he said.
He said high-level Afghan officials, including President Hamid Karzai's brother, a minister and a deputy minister, were all attending the funeral and may have been the target of the firing.
Another villager in the area said he heard a large explosion followed by gunfire.
Afghanistan's parliament has demanded a public trial for the suspect, and the Afghan Taliban have described U.S. troops as "sick-minded American savages" and vowed to exact revenge. In a new statement Tuesday, the Taliban said the group would take revenge "by killing and beheading Americans anywhere in the country."
The suspect has not been charged. The Army's Criminal Investigation Command is leading the investigation, and U.S. officials say the case will be handled by the American military.
There are fears that Sunday's killings could reignite the anger that led to deadly riots directed at international forces last month over the burning of Qurans by U.S. troops. That was one of a string of incidents involving American forces that have strained ties between the United States and Afghanistan.
U.S. commanders were forced to condemn 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."
Sunday's killings have brought a deluge of high-level statements from Washington expressing shock, sadness and insistence that the U.S. mission in Afghanistan would stay on course.
As he set out for a trip to the Middle East on Monday, 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 still-unidentified suspect in the attack served three tours of duty in Iraq before being deployed to Afghanistan, said Gen. John Allen, the U.S. and NATO commander in Afghanistan. A U.S. military official, who asked not to be named because he was talking about an ongoing investigation, said the suspect is an Army staff sergeant who arrived in Afghanistan in January.
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. The sergeant was diagnosed with a traumatic brain injury after the wreck but was found fit for duty after treatment, the official said.
CNN's Jethro Mullen and journalist Ruhullah Khapalwak contributed to this report.