CAIRO -- Egyptian rights activists have accused guards at a Cairo prison of torturing a prisoner to death.

The authorities' use of torture was a key grievance behind the popular uprising that overthrew President Hosni Mubarak in February.

Lawyer Malik Adly said Friday that a prison guard brought 23-year-old Essam Atta to a Cairo hospital, where he was later pronounced dead from "unknown poisoning."

Adly said Atta recently told his family that guards tortured him by using tubes to inject water into his mouth and anus.

An Egyptian security official denied the allegations, saying prison medics found that Atta had taken drugs and was suffering from exhaustion, so he was taken to the hospital.

The official spoke on condition of anonymity because he was not authorized to brief the media.