Bahrain executed three men on Saturday, including two Shiite activists for what officials called “terrorism crimes,” in what was described as attacks orchestrated by Iran-based ringleaders.

A London-based Bahraini activist rights group said the executions made Saturday “one of Bahrain’s darkest days.”

The men were convicted in two separate cases, one involving the killing of a police officer in 2017 and the other the killing of an imam in 2018, the public prosecutor said in a statement.

The Shiite activists, Ali al-Arab and Ahmed al-Malali, were sentenced to death last year as part of a mass trial on “terrorism crimes.” Nineteen men were jailed for life, and 37 others for terms of up to 15 years, on the accusation that they had belonged to a terrorist cell trained to use heavy weapons and explosives.