Police said that the four – three Spaniards and one Moroccan -- were "willing to commit terrorist acts on Spanish soil." Authorities also said that they were already in the process of procuring weapons and substances to make bombs.
The four are also suspected of recruiting for the Islamic State -- particularly minors.
Police said of the former Gitmo-detainee, who was not named, "The fact that this leader was trained in handling weapons, explosives and in military tactics makes this cell particularly dangerous."
The recidivism rate of former Gitmo-detainees is high. Senator John McCain, chair of the Senate Armed Services Committee, put the number at 30%, although other’s claimed it was lower.
According to a September 2014 report by the Office of the Director of National Intelligence, of the 620 detainees transferred out of Gitmo, 107 had been "confirmed of re-engaging" and 77 were "suspected of re-engaging" in terrorist or insurgent activities.
On Tuesday, February 23, 2016, U.S. President Barack Obama called for a closure of the facility. "This is about closing a chapter in our history. Keeping this facility open is contrary to our values. It undermines our standing in the world. It is viewed as a stain on our broader record of upholding the highest standards of rule of law," the president said.
The following are notable detainees at Gitmo that have been recently freed:
Egyptian Tariq Mahmoud Ahmed al Sawah , one of al Qaeda’smost skilled explosives experts, was released in January 2016. Al Sawah was personally praised by Osama bin Laden, may have known of the original Sept. 11 plot and was the creator of the shoe-bomb with which a jihadi tried to down a commercial airplane in 2001.
Mohammad Al Rahman Al Shumrani, a 40-year old Saudi Arabian national, was released on January 11, 2016, even though he had vowed to kill “as many Americans as possible.” The previous August, he was listed as a “forever prisoner” – too dangerous to release.
Ibrahim Ahmed Mahmoud al Qosi was released from Gitmo and transferred to Sudan in July, 2012. In December, 2015, heappeared as a spokesman in a video for Al Qaeda in the Arabian Peninsula.
The “Taliban Five,” all high-ranking members of the Afghan Taliban and considered “high risk” and dangerous by the U.S, were released on June 1, 2014 in exchange for Bowe Bergdahl, a U.S. soldier who was later charged with desertion.