In apparent retaliation over the burning of religious books at a disposal site at Bagram base, two American soldiers were shot and killed and four others were wounded when an Afghan soldier turned his weapon on them and opened fire today.

Violent protests over the burning have been prevalent throughout Afghanistan for the last three days, they continue on the heels of apologies from U.S. and coalition leaders over the unfortunate incident. Ashton Carter, the United States Department of Defense Deputy Secretary, arrived there earlier this week and has joined in the condemnation of the burning. Carter said that it was inadvertent and should not have happened. “It’s inexcusable — an act of ignorance — and it’s very unfortunate because it has unfortunate consequences and gives people a chance to misunderstand what we’re all doing here,” he said.

In meetings yesterday with Afghan President Hamid Karzai, Defense Minister Abdul Rahim Wardak and members of the Afghan parliament, Carter offered his personal apologies for the incident. In each meeting, the deputy defense secretary made it clear these were “ignorant” actions that were unacceptable.

Speaking from the Afghan capital of Kabul in a teleconference with Pentagon reporters, Brig. Gen. Carsten Jacobson of the German army said it’s of the utmost importance that officials “explain very, very clearly what happened, explain how sorry we are about what happened, explain that this was a mistake, explain what led to it, and talk about the consequences.”

Jacobson said the Qurans were mistakenly included in a mass of material delivered to the detention facility’s burn pit for routine disposal. Local workers at the facility noticed the books and pulled them from the fire, he added.

“Material was inadvertently given to troops for burning,” Jacobson said. “The decision to burn this material had nothing to do with it being religious in nature or related to Islam. It was a mistake. It was an error.”

Marine Corps Gen. John R. Allen, ISAF commander, quickly ordered an investigation to discover how and why the mistake happened, the spokesman said.

More backlash in the burning incident is expected on Friday, at the end of holy week in the country. That is when many of the young men in the country are expected to take to the streets to continue to protest.

Afghan President Hamid Karzai’s office said Thursday that President Obama had sent a letter to him apologising for the incident.