Over 14 inmates were killed and dozens were injured when a building housing at least 1300 prisoners was destroyed in a bomb attack in Adigrat town on Wednesday, a source said.
Bulldozers were used to pull out bodies from the debris. Those critically injured were admitted to the hospital in town. Fears have mounted that the death toll may rise.
The government has remained quiet.
Meanwhile, rebels of the Ethiopian Unity and Freedom Force (EUFF) have claimed responsibility for the attack. However, their claim couldn't be verified by an independent source.
An EUFF spokesperson told Ethiomedia by phone that the action was taken to free political prisoners that the government had kept behind bars since the ill-fated 2005 elections in which the opposition Kinijit was believed to have won.
Most inmates were Kinijit supporters arrested from Gonder, Gojjam, Addis Ababa and other parts of the country during a brutal government crackdowns in the aftermath of the 2005 elections, the source said. "Demands that the government release the political prisoners had fallen on deaf ears, thus prompting a public demand that the rebels take action."
Over 40 prisoners have remained at large, according to the spokesperson. A climate of fear and uncertainty has engulfed the once-bustling town of Adigrat.
Unlike other rebel groups that claim support from Ethiopia's arch enemy Eritrea, the fiercely-independent EUFF has been active in northern Ethiopia in recent years.
Last April, the rebels set on fire a business district in Metema town on the border with Sudan. Over 60 business units that belonged to the ruling party officials were razed to the ground.