Survivors of a massacre by rebels in western Ethiopia on Sunday counted 54 bodies in a schoolyard, the latest attack in which members of ethnic minorities have been deliberately targeted, Amnesty International said Monday.

Human rights groups are asking why federal soldiers left the area just hours before attackers moved in and targeted ethnic Amharas.

Ethiopia’s prime minister, Abiy Ahmed, denounced the killing of people based on identity, adding that security forces had been deployed to the area and “started taking measures.”

Ethnic violence in Ethiopia is posing the greatest challenge yet to the prime minister, who was last year’s Nobel Peace Prize winner for his sweeping political reforms.

Read more: Washington Post