Pentagon and military officials, noting that the proliferation of soldiers’ carrying camera phones has been involved in many of the cases, said that technology and a changing culture had presented new problems, as well. Troops have behaved badly since the beginning of warfare, of course. But now, those actions can be captured in real time, and spread rapidly without commanders’ control, via social networks.

Army officials said Wednesday that the service had guidelines and rules for photos — basically, “think before you post” — but they also acknowledged that social media are evolving so rapidly that regulations were not keeping pace. Rules are set by commanders at the company, battalion and brigade level, but those standards are sometimes ignored by small units in the field.

“Technology today presents definite challenges related to security and propriety,” said Col. Thomas W. Collins, an Army spokesman. “In this case, these photos are probably a manifestation of the soldiers’ relief that this insurgent no longer posed a threat to them or their fellow soldiers. That cannot excuse what they did. We are the United States Army, and the world rightly has very high expectations that our soldiers will do what’s right. Clearly, that didn’t happen in this case.”

With more than a million military personnel having deployed overseas since the Sept. 11 terrorist attacks, the recent cases represent only a tiny percentage of the force. Senior American officials responding on Wednesday noted that, even as they condemned the soldiers’ behavior.

“This is not who we are, and it’s certainly not what we represent when it comes to the great majority of men and women in uniform,” Defense Secretary Leon E. Panetta said at a NATO conference in Brussels, calling the soldiers’ behavior unacceptable and promising a full investigation. President Obama said that those responsible for the actions would be “held accountable,” and Gen. John R. Allen, the senior allied commander in Afghanistan, sounded similar themes. But Afghan officials described an increasing skepticism among the public after case after case of misbehavior has come to light over the past year.

Nadir Nadiry, an Afghan human rights activist in Kabul, said Afghans would likely react negatively because similar photographs had surfaced before and despite military investigations the latest pictures suggested that the actions continued to be perpetrated. “It gives them a sense of, ‘Oh they are continuing to do this,’ ” he said. “Each time they say they will conduct a thorough investigation, but these investigations are not being made public, so the results are not known to the Afghan people.”