Gitmo

It seems some prisoners at Guantanamo Bay are on a hunger strike. That's not a big deal, but I noticed:

The Pentagon's version of this incident contrasted somewhat from the accounts of two Afghans released from the facility for terrorist suspects earlier this week. On Wednesday, they claimed that more than about 180 Afghans were on a hunger strike to protest alleged mistreatment at the facility at a U.S. military base in Cuba.

I'm a bit confused. I thought they were terra-ists, or Taliban. Weren't they in Gitmo because they were a danger to America? So why were they released? Where they innocent? If they were innocent, why were they incarcerated for 4 years?