Doctors Without Borders (known internationally as Médecins Sans Frontières) has suspended its treatment activities in the epicenter of an Ebola outbreak in the Democratic Republic of the Congo after two of its treatment facilities were violently attacked in the past four days, according to a press release from the organization. On Feb. 24, unidentified assailants threw stones at a treatment facility in Katwa, and set parts of the structure on fire. On Feb. 27, parts of a treatment center in Butembo were also set ablaze. At the time, the organization notes, 57 patients were in the center—including 15 people with confirmed cases of Ebola. No one was hurt, and the patients have been transferred to a new center.

“In light of these two violent incidents, we have no choice but to suspend our activities until further notice,” emergency desk manager Hugues Robert said in the press release. “As medical responders, it is very painful to have to leave behind patients, their families, and other members of the community at such a critical time in the Ebola response.” The release notes that since the outbreak began in August, there have been 879 confirmed cases and 553 people have died. It remains unclear exactly who is behind the attacks: The epicenter is an active war zone with fighting between numerous militia groups, and community members often distrust medical workers.