At least two are believed to have died in fighting in Afghanistan, and a third was recaptured during a raid of a suspected training camp in Afghanistan, said Lt. Cmdr. Flex Plexico, a Pentagon spokesman. Others are at large.
Additional former detainees are said to have expressed a desire to rejoin the fight, be it against U.N. peacekeepers in Afghanistan, Americans in Iraq or Russian soldiers in Chechnya.
Some 146 detainees have been released from Guantanamo, but only after U.S. officials had determined the prisoners no longer posed threats and had no remaining intelligence value.
Pentagon officials acknowledged that the release process is imperfect, but they said most of the Guantanamo detainees released have steered clear of Islamic insurgent groups.
The small number returning to the fight demonstrates the delicate balance the United States must strike between minimizing the appearance of holding people unjustly and keeping those who are legitimate long-term threats, Pentagon spokesman Bryan Whitman said.
Human rights groups frequently criticize the Defense Department for holding the hundreds of prisoners at the naval base, largely without charges or legal counsel. Many have been held for more than two years; only a few have been charged.
An additional 57 Guantanamo prisoners have been transferred to the custody of their home governments: 29 to Pakistan; seven to Russia; five each to Morocco and Britain; four each to France and Saudi Arabia; and one each to Spain, Sweden and Denmark, the Pentagon has said.
The Pentagon did not identify the seven detainees believed to have returned to fighting, although a few names have been made public. One released detainee killed a judge leaving a mosque in Afghanistan, Plexico said.
Those in the small group that has gone back to fighting come mainly from the upper echelons of suspected militant or terror groups, some allegedly linked to al Qaeda, several counterterrorism officials in the Middle East said. They gave no details, but one noted a trend that lower-echelon members tend to get on with their lives after they are released.
The former prisoners include Abdullah Mehsud, a former Guantanamo Bay detainee linked to al Qaeda who oversaw the recent kidnapping of two Chinese engineers, one of whom was killed.
On Friday, Pakistani soldiers began a massive search for Mehsud, 28, who returned to Pakistan in March after about two years' detention at Guantanamo. Pakistan officials say he has forged ties with al Qaeda since then.
One of the two former prisoners killed is Maulvi Abdul Ghaffar, a senior Taliban commander in northern Afghanistan who was arrested about two months after a U.S.-led coalition drove the militia from power in late 2001.
He was held at Guantanamo for eight months, then released, and was killed about a month ago, on Sept. 26, by Afghan security forces during a raid in Uruzgan province. Afghan leaders said they believed he was leading Taliban forces in the southern province.
Maj. Gen. Eric Olson, the No. 2 commander of U.S. troops in Afghanistan, told The Associated Press this month there was no alternative to releasing prisoners from Guantanamo.
"It's not going to be perfect, so it (the Ghaffar case) has not led to any soul-searching about the release program," Olson said.
Other former prisoners have expressed publicly they wanted to return to the fight.
In Denmark, Slimane Hadj Abderrahmane, who was released in February from the U.S. naval base on Cuba's southeastern tip, said he would go to Chechnya to fight with rebels there against Russia.
"The Muslims are oppressed in Chechnya, and the Russians are carrying out terror against them," the 31-year-old Dane, who has an Algerian father, told Danish television in September.
Abderrahmane, who was never charged in Denmark upon his return, later backtracked. After being questioned by Danish intelligence agents, he said he would stay in Denmark, hand over his passport and honor his pledge. Danish intelligence officials are keeping tabs on Abderrahmane.
In Sweden, Mehdi-Muhammed Ghezali, who was released in July after more than two years at the base, is being monitored by Swedish intelligence agents. While Sweden's security police, SAPO, has no official comment, its agents have said Ghezali is not a threat.
Other former Guantanamo prisoners, including Yaser Esam Hamdi of Saudi Arabia, had their releases held up amid fears they would rejoin their comrades.
Hamdi, who was born in Louisiana, spent three years in solitary confinement, first at Guantanamo and then at a Navy brig in South Carolina after he was captured in Afghanistan in 2001. He was returned to Saudi Arabia on Oct. 11 after agreeing to forfeit his U.S. citizenship.
He also is required to stay in Saudi Arabia for five years, renounce terror and cannot travel to Afghanistan, Iraq, Israel, Pakistan or Syria. Additionally, Hamdi must notify Saudi officials if he becomes aware of "any planned or executed acts of terrorism."
It is likely that Hamdi will be monitored by government officials there, as much as Ghezali and Abderrahmane have been in northern Europe.