An American working for an evangelical aid group has tested positive for Ebola in the Democratic Republic of Congo, where aid groups are struggling to contain a surging outbreak that has already killed over 600 people.

The U.S. Centers for Disease Control and Prevention said on Friday the aid worker had contracted Ebola from the Bundibugyo virus, which is driving the current outbreak, already the third largest on record. Bundibugyo has no licensed vaccine or treatment, prompting a scramble by international researchers to develop one.

The affected American worked for Samaritan’s Purse, a disaster relief organization headed by the evangelist Franklin Graham, a spokesman for the group, confirmed in an email. The affected worker has been in isolation since Monday, the spokesman said, and is being cared for at one of two Ebola treatment centers the organization runs in Ituri, the province at the center of the current outbreak in northeastern Congo.

The group did not identify the affected employee but said he had been helping with logistics in the regional capital, Bunia, for the past month, and was not involved in direct patient care.

The aid worker is the second American to contract Ebola in this outbreak, after a missionary doctor tested positive soon after the outbreak was declared on May 15, and was later evacuated to a hospital in Germany.

How to deal with American Ebola patients is becoming a vexing issue for the Trump administration, which has sought to break with previous administrations’ approach of bringing health care workers and other U.S. citizens exposed to the virus home to be treated at specialized medical units.