Among the 10 Yemeni terrorists released by the Obama administration this week, one is Muhammad Salih Husayn al-Shaykh, a man who has pledged to kill as many Americans as possible.

Another of the latest group is Abu Bakr Ibn Muhammad al-Ahdal, an al Qaeda member described by the intelligence community as a “willing terrorist against the U.S.”

In fact, five of the 10 were deemed by the George W. Bush administration in 2008 as a “high risk” to go back on the battlefield and kill Americans. The other five were designated “medium” risks.

The release of the 10 detainees brings the population at Guantanamo Bay, Cuba, to 93 — the first time it’s dropped below 100 detainees since 2002, at the height of the fight in Afghanistan against al Qaeda and the Taliban.

Defense Secretary Ashton Carter also said Thursday he’s “framed” for President Obama a plan to close the prison entirely and bring the remaining detainees to the U.S. He said he hopes Congress will approve the plan, but, in the meantime, he’ll keep looking for cases such as al-Shaykh and al-Ahdal.

“While we work with Congress on the way forward, we will continue to transfer Guantanamo detainees to other countries when and as we have mitigated any security risk to the United States,” he said.

But the release of high-risk detainees to Oman, from which they could relaunch terror careers in nearby Yemen, underscores the increasingly slim pickings Mr. Obama and Mr. Carter have as they press for shuttering the prison.

Sen. Kelly Ayotte, New Hampshire Republican, said she fears the 10 will spend little time in Oman before traveling home to Yemen, where al Qaeda in the Arabian Peninsula operates and is determined to strike the American homeland.

“I am particularly concerned that the Omani government has accepted the detainees for what they are calling a ‘temporary stay,’ which raises serious questions about whether they will even be detained in Oman, for how long and under what conditions,” Ms. Ayotte said. “The administration should be forthright with the American people regarding these concerns, and also about the detainees’ terrorist activities, associations and their actions at Gitmo.”

Ibrahim al-Qosi, a bin Laden confidant released by the Obama administration in 2012, has returned to terrorism as a spokesman and recruiter for AQAP.

About 30 percent of Guantanamo graduates have resumed, or are suspected of restarting, terrorist activity. The number will likely increase as intelligence agencies gather more information. The Obama administration points out that, if they do reengage, they are subject to being killed.

The public knows about these terrorists, not because the government released details, but because WikiLeaks acquired and posted scores of “secret” 2008 detainee assessments by Joint Task Force Guantanamo.

The dossiers are chilling in that the al Qaeda members seemed just as committed in 2008 as when they joined Osama bin Laden’s al Qaeda in the Afghanistan-Pakistan nexus.

Al-Shaykh presented himself to U.S. officials as a hardened member of al Qaeda who wants to kill.

“Detainee has threatened to kill all Americans and identified [bin Laden] as his brother in arms,” the assessment states. “Detainee was recruited through a known al Qaeda facilitation network and he was identified as a suicide operative. Detainee’s threats to kill U.S. personnel and his refusal to truthfully answer questions indicate his continuing support to extremism.”

The assessment depicted al-Shaykh as a liar, the user of many aliases and someone deeply rooted in the al Qaeda culture of murder justified by the Koran.

“Detainee has repeatedly threatened to kill US personnel,” reads another report section. “Such threats include the statement that he will kill all Americans and specific threats to cut off the head of guard personnel and his statement, ‘I will kill you, Captain, and I will take your religion away from you.’ Detainee also expressed the threat that [bin Laden] will kill the General (former CJTF), and the president.”

For al-Ahdal, the assessment said, “Detainee identified himself as a willing terrorist against the U.S. and acknowledged he was a member of a terrorist support entity in Yemen.”

Al-Ahdal fought with al Qaeda in northern Afghanistan and in Tora Bora, one of the last major battles of the 2001 U.S. invasion during which bin Laden slipped away. He was killed in a May 2011 raid in Pakistan by U.S. Navy SEALs.

Al-Ahdal was a member of bin Laden’s infamous 55th Arab Brigade, which formed an alliance with the Taliban then ruling Afghanistan, helping them to stay in power.

Three other inmates released to Oman were also previously judged too high a threat risk to be released.

• Adham Mohamed Ali Awad: An al Qaeda member, Awad made statements “indicating a HIGH probability for recidivist activities including suicide attack against large numbers of both military and civilian population.”

• Samir Naji al-Hasan Muqbil: He served on bin Laden’s personal security detail.

• Fahmi Abdallah Ahmad Ubadi al-Tulaqi: He was both a member of al Qaeda and the Taliban and fought in bin Laden’s Arab brigade. He was a member of a Pakistani terror group committed to building bombs to kill American troops.

These were 2008 assessments. The Obama administration views the five hardened terrorists differently.

A Pentagon press statement said the interagency Guantanamo Review Task Force “conducted a comprehensive review of these cases” and “were unanimously approved for transfer by the six departments and agencies comprising the task force.”

The statement added, “The United States is grateful to the government of Oman for its humanitarian gesture and willingness to support ongoing U.S. efforts to close Guantanamo Bay detention facility.”

Guantanamo’s population peaked at 779, and has now dropped to 93.

Among the detainees released this week is Fahd Ghazy, who was 17 when he was brought to the prison in February 2002. He was never charged with a crime, and was cleared for release as far back as 2007, his lawyers said.

“There was never much doubt that Fahd’s imprisonment was unnecessary — he was cleared for release nearly a decade ago — yet he grew up at Guantanamo waiting for successive presidents to correct a glaring injustice,” said Omar Farah of the Center for Constitutional Rights, who is Mr. Ghazy’s lawyer.

Mr. Obama had promised to close Guantanamo within a year of taking office in 2009, but Congress has repeatedly passed laws banning him from bringing detainees to the U.S. Mr. Obama has signed each of those laws.

As of the end of this month, the prison will have been open longer under Mr. Obama than it was under President Bush, the Center for Constitutional Rights said.

House Speaker Paul D. Ryan tried to press Mr. Obama on his Guantanamo plans as the president conducted a Twitter town hall Thursday.

“What authority do you have to close Gitmo? Even Democrats supported restrictions that you signed into law,” the Wisconsin Republican tweeted to the president.

Mr. Obama ignored the question.