The fate of three of nine foreigners abducted in Yemen last week is known — their bodies were found, shot execution style. The whereabouts of the other six — including three children under the age of 6 — remain a mystery.

But terrorism experts say their abductors and killers are almost certainly not a mystery. They say the crimes bear the mark of Al Qaeda, and they fear they are the handiwork of the international terror organization's No. 2 man in the Arabian Peninsula: Said Ali al-Shihri, an Islamic extremist who once was in American custody — but who was released from the U.S. detention center in Guantanamo Bay, Cuba.

And if al-Shihri is behind the gruesome murders and abductions, they say, it raises grave concerns that the scheduled January 2010 closing of the Guantanamo prison and the release of most of its prisoners to foreign countries will galvanize Al Qaeda and compromise American national security.

The nine foreigners — four German adults, three small German children, a British man and a South Korean woman — were abducted on June 12 after they ventured outside the city of Saada without their required police escorts, according to a spokesman from the Yemeni Embassy in Washington. Days later the bodies of Rita Stumpp and Anita Gruenwald, German nurses in training, and Eom Young-sun of South Korea were found shot execution style in the Noshour Valley in the province of Saada, an area known to be a hotbed of Al Qaeda activity.

Stumpp and Gruenwald attended a Bible school, and Young attended a Christian missionary school in South Korea. Other members of the group had ties to missionary organizations, and all six adults worked for World Wide Services Foundation, a Dutch international medical relief group.

No one has claimed responsibility for the abductions and murders, but experts say killing women and children is considered off-limits among many jihadist groups — though not to al-Shihri, a Saudi national who was released from Guantanamo in November 2007 and sent to a Saudi Arabian "rehabilitation" program for jihadists. It wasn't long before a "cured" al-Shihri was released from the program, crossed into Yemen and rejoined Al Qaeda, with whom he quickly rose to deputy commander.

In addition to last week's kidnappings, he is believed to have been behind the September attacks that left 16 dead at the U.S. Embassy in the Yemeni capital of San'a.

“This bears the marks of al-Shihri’s activity and bears the signs of his beliefs and assumptions of his behavior that are not viewed by other jihadists,” said Robert Spencer, terror expert and director of Jihad Watch, referring to the killing of women and presumed killing of the three small children.

Defense officials said that "while there is suspicion that Said Ali al-Shihri is involved in these latest attacks, we can't confirm it."

“There is great presumption of his involvement, but it’s not open and shut,” Spencer said.

“If he believed that these people picnicking in Yemen were aiding in the war against Islam, then he can justify these killings as legitimate — it’s this kind of perspective that this guy holds to, that it’s right to kill people who would normally be considered off-limits,” Spencer said.

“Christians aren’t allowed to proselytize in the Muslim world, and if that’s what was going on here ... well then, there you go.”

Gregory Johnsen, the editor of a forthcoming book, "Islam and Insurgency in Yemen," agreed. “The most likely scenario is that Al Qaeda’s responsible," he said. "And if it does turn out that Al Qaeda is responsible, then it would be that al-Shihri had a hand in the operation whether behind the scenes or up front.”

And that involvement is an ominous sign for critics, who say the release of detainees from Guantanamo, under President Obama's plan to close the detention center by January 2010, could lead to future and more severe terrorist attacks.

The United States is negotiating with Yemen, according to Mohammed Albasha, spokesman for Embassy of the Republic of Yemen in Washington, over the possible transfer of the more than 100 Yemeni nationals currently behind bars in Gitmo.

Rep. Peter King (R-NY), Ranking Member on the House Homeland Security Committee, opposes closing Gitmo and says Obama is rushing "helter skelter" to find homes for the remaining detainees to meet his "arbitrary deadline," which he says may come at the cost of national security.

“The president’s policies are very very disturbing. He appears to have decided to close Guantanamo without any idea of where these detainees are going to go and is now trying desperately to find countries and places for these people to go,” King told FOXNews.com.

“By far largest number of detainees comes from Yemen, and they are hardcore dangerous people. Sending them back to Yemen, where prisoners who have been held there before somewhere magically escaped from prison, and is in many ways the centerpiece of the Al Qaeda movement — returning them to Yemen is just inviting disaster.

"Sending them to Saudi Arabia to the rehabilitation center is just putting off the inevitable threat to the United States,” King said. “I am very concerned that these prisoners will soon be back in the battlefield hurting Americans.”