As many as 20 to 30 former Guantanamo Bay detainees released within the last two to three years are suspected by intelligence and Defense officials of having joined forces with the Islamic State and other militant groups inside Syria, Fox News has learned. 

The development has cemented fears that the U.S. military would once again encounter militants taken off the battlefield. 

The intelligence offers a mixed picture, and officials say the figures are not exact. But they are certain at least some of the released detainees are fighting with the Islamic State, or ISIS, on the ground inside Syria. Others are believed to be supporting Al Qaeda or the affiliated al-Nusra Front in Syria. 

A number of former detainees also have chosen to help these groups from outside the country, financing operations and supporting their propaganda campaigns.