Two attacks on Ebola treatment centers in the Democratic Republic of Congo have forced the international aid group Doctors Without Borders to close the facilities, it said on Thursday, warning that the outbreak was not under control.

In the first attack, on Sunday night, unidentified assailants threw stones at a treatment center in Katwa, in northeastern Congo, and set fire to the structure. They destroyed much of the building, equipment and patient wards, and the brother of a patient died, though the circumstances were unclear.

Doctors Without Borders said that its staff members were safe and that patients had been transferred to other facilities, but it called the attack a debilitating blow to efforts to contain the outbreak, which has killed 553 people so far. It is the second-worst Ebola epidemic in history, after the crisis that left 11,310 people dead in West Africa from 2014 through mid-2016.

“This attack has crippled our ability to respond to what is now the epicenter of the outbreak,” Emmanuel Massart, the emergency coordinator for Doctors Without Borders in Katwa, said in a statement on Tuesday.