BEIRUT, Lebanon — The emergence of a former Guantánamo Bay detainee as the deputy leader of Al Qaeda’s Yemeni branch has underscored the potential complications in carrying out the executive order President Obama signed Thursday that the detention center be shut down within a year.

The militant, Said Ali al-Shihri, is suspected of involvement in a deadly bombing of the United States Embassy in Yemen’s capital, Sana, in September. He was released to Saudi Arabia in 2007 and passed through a Saudi rehabilitation program for former jihadists before resurfacing with Al Qaeda in Yemen.

His status was announced in an Internet statement by the militant group and was confirmed by an American counterterrorism official.

“They’re one and the same guy,” said the official, who insisted on anonymity because he was discussing an intelligence analysis. “He returned to Saudi Arabia in 2007, but his movements to Yemen remain unclear.”

The development came as Republican legislators criticized the plan to close the Guantánamo Bay, Cuba, detention camp in the absence of any measures for dealing with current detainees. But it also helps explain why the new administration wants to move cautiously, taking time to work out a plan to cope with the complications.

Almost half the camp’s remaining detainees are Yemenis, and efforts to repatriate them depend in part on the creation of a Yemeni rehabilitation program — partly financed by the United States — similar to the Saudi one. Saudi Arabia has claimed that no graduate of its program has returned to terrorism.

“The lesson here is, whoever receives former Guantánamo detainees needs to keep a close eye on them,” the American official said.

Although the Pentagon has said that dozens of released Guantánamo detainees have “returned to the fight,” its claim is difficult to document, and has been met with skepticism. In any case, few of the former detainees, if any, are thought to have become leaders of a major terrorist organization like Al Qaeda in Yemen, a mostly homegrown group that experts say has been reinforced by foreign fighters.

Long considered a haven for jihadists, Yemen, a desperately poor country in the southern corner of the Arabian Peninsula, has witnessed a rising number of attacks over the past year. American officials say they suspect that Mr. Shihri may have been involved in the car bombings outside the American Embassy in Sana last September that killed 16 people, including six attackers.

In the Internet statement, Al Qaeda in Yemen identified its new deputy leader as Abu Sayyaf al-Shihri, saying he returned from Guantánamo to his native Saudi Arabia and then traveled to Yemen “more than 10 months ago.” That corresponds roughly to the return of Mr. Shihri, a Saudi who was released from Guantánamo in November 2007. Abu Sayyaf is a nom de guerre, commonly used by jihadists in place of their real name or first name.

A Saudi security official, speaking on the condition of anonymity, said Mr. Shihri had disappeared from his home in Saudi Arabia last year after finishing the rehabilitation program.

A Yemeni journalist who interviewed Al Qaeda’s leaders in Yemen last year, Abdulela Shaya, confirmed Thursday that the deputy leader was indeed Mr. Shihri, the former Guantánamo detainee. Mr. Shaya, in a phone interview, said Mr. Shihri had described to him his journey from Cuba to Yemen and supplied his Guantánamo detention number, 372. That is the correct number, Pentagon documents show.

“It seems certain from all the sources we have that this is the same individual who was released from Guantánamo in 2007,” said Gregory Johnsen, a terrorism analyst and the editor of a forthcoming book, “Islam and Insurgency in Yemen.”

Mr. Shihri, 35, trained in urban warfare tactics at a camp north of Kabul, Afghanistan, according to documents released by the Pentagon as part of his Guantánamo dossier. Two weeks after the terrorist attacks of Sept. 11, 2001, he traveled to Afghanistan via Bahrain and Pakistan, and he later told American investigators that his intention was to do relief work, the documents say. He was wounded in an airstrike and spent a month and a half recovering in a hospital in Pakistan.

The documents state that Mr. Shihri met with a group of “extremists” in Iran and helped them get into Afghanistan. They also say he was accused of trying to arrange the assassination of a writer, in accordance with a fatwa, or religious order, issued by an extremist cleric.

However, under a heading describing reasons for Mr. Shihri’s possible release from Guantánamo, the documents say he claimed that he traveled to Iran “to purchase carpets for his store” in Saudi Arabia. They also say that he denied knowledge of any terrorists or terrorist activities, and that he “related that if released, he would like to return to Riyadh, Saudi Arabia, wherein he would reunite with his family.”

“The detainee stated he would attempt to work at his family’s furniture store if it is still in business,” the documents say.

The Yemeni branch of Al Qaeda has carried out a number of terrorist attacks over the past year, culminating in the assault on the American Embassy in Sana on Sept. 16. In that assault, the attackers disguised themselves as Yemeni policemen and detonated two car bombs. The group has also begun releasing sophisticated Internet material, in what appears to be a bid to gain more recruits.

Yemen began cooperating with the United States on counterterrorism activities in late 2001. But the partnership has been a troubled one, with American officials accusing Yemen of paroling dangerous terrorists, including some who were wanted in the United States. Some high-level terrorism suspects have also mysteriously escaped from Yemeni jails. The disagreements and security lapses have complicated efforts to repatriate the 100 or so Yemenis remaining in Guantánamo.

Despite some notable Yemeni successes in fighting terrorist groups, Al Qaeda in Yemen appears to be gaining strength.

“They are bringing Saudi fighters in, and they want to start to use Yemen as a base for attacks throughout region, including Saudi Arabia and the Horn of Africa,” said Mr. Johnsen, an expert on Al Qaeda in Yemen.