Detainees, some innocent, but many of them former insurgents long held in American military custody, are being set free every day, potentially increasing the insurgency’s numbers. The American-Iraqi security agreement requires the release of all detainees in American custody unless there is sufficient evidence to bring charges in an Iraqi court.

At least one former detainee has already blown himself up in a suicide attack.

Most of the latest attacks, at a time when overall violence is at its lowest level since the beginning of the war in 2003, have singled out Iraqis, but one development affects the Americans. A new weapon has appeared in Iraq: Russian-made RKG-3 grenades, which weigh just five pounds and, attached to parachutes, can be lobbed by a teenager but can penetrate the American military’s latest heavily armored vehicle, the MRAP. The grenades cost as little as $10, according to American military officials, who would not say how often they have killed soldiers.

To some experts, this amounts to ugly, but unavoidable, background noise, the deadly but no longer destabilizing face of violence in Iraq. In this view, there will be attacks, but no longer ones likely to topple Iraq’s government. Military officers, who spoke on the condition of anonymity because they were not authorized to talk to the news media, say they have reduced the number of jihadi militants to under 2,000, from about 3,800.

“In most places there isn’t an insurgency in Iraq anymore,” said an American military intelligence officer in Washington, who was not authorized to be quoted by name. “What we have now is a terrorism problem, and there is going to be a terrorism problem in Iraq for a long time.”

Other officials, Iraqi and American, are more worried. They observe jihadi and other insurgent groups activating networks of sleeper cells, which are already striking government and civilian targets. Insurgent groups linked to the rule of Mr. Hussein are also reviving.