A former Guantanamo Bay detainee identified as Lahcen Ikassrien was among those arrested in Spain over the weekend for allegedly recruiting on behalf of a brutal terrorist group, the Islamic State of Iraq and Syria.

Ikassrien's arrest raises concern about the activities of other released detainees, particularly the five high-risk detainees released by the Obama administration as part of a prisoner swap in May.

Ikassrien was arrested along with nine other suspected Islamic militants in Madrid. Spanish authorities claim the 10 individuals were recruiting jihadis to fight for ISIS in the Middle East.

Spain is a fertile recruiting ground for terrorist organizations, especially southern Spain, which is just across the Mediterranean from Morocco. Spanish authorities have unearthed multiple terrorist recruiting cells this year alone.

Ikassrien, who allegedly led the recruiting cell, was arrested in 2001 in Kunduz, Afghanistan, where he was fighting for the Taliban. He spent four years in custody at Guantanamo Bay before facing trial in Spain. He was released in 2006 when it was determined he was not a member of al Qaeda.

Now we may know how Ikassrien spent the intervening years between his release and arrest in Spain — as a headhunter for ISIS.

ISIS is a Sunni terrorist group that seeks to create a unified Islamic state in place of the region's current nations and governments, which it views as illegitimate and heretical. While ISIS formerly partnered with other radical groups like al Qaeda and its Syrian affiliate Jabhat al-Nusra, it broke with both groups this year after intense bickering.

ISIS is the cause of recent turmoil in Iraq, which has jeopardized the Iraqi government and reversed the gains of a long U.S. occupation. It currently reigns over a quasi-state in northern Iraq that includes rich oil fields, weapons depots and major cities like Mosul. Allegations of its atrocities, which include mass beheadings, are legion.

If true, Ikassrien's decision to rejoin the fight could foreshadow events to come. Last month the Obama administration freed five Taliban commanders from Guantanamo Bay in a prisoner swap for Army Sgt. Bowe Bergdahl, who was held in Iraq.

While Ikassrien is small fry, the "Taliban Five" were deemed high risks to the United States by the Joint Task Force Guantanamo. Officials testified before Congress that four of the five commanders will likely rejoin the fight -- one has even pledged to family members that he will return to Afghanistan to fight U.S. troops stationed there.

While the administration and its defenders have -- with the notable exception of former Secretary of State Hillary Clinton -- conceded that the Taliban Five could go back to targeting the U.S., they have defended the decision to release them. Defense Secretary Chuck Hagel justified the decision in part because none of the commanders have been implicated in attacks against the United States.

Then again, neither has Ikassrien — so far.