BUNIA, Congo (AP) — Confirmed cases in the Ebola outbreak in eastern Congo have reached 1,003, including 254 deaths, officials said in a statement late Sunday, as tracing those who had been in contact with patients remains a major challenge.

A total of 100 people have recovered in the outbreak concentrated in the Ituri province since it was declared on May 15, Congo’s Ministry of Health said. At least 365 patients are in hospitals or in isolation, it said.

The Ebola outbreak caused by the rare Bundibugyo virus, which has no vaccines or treatment, was the worst ever in its first month. Officials admit there could be far more cases they still don’t know about and that the peak of the outbreak is still ahead.

Contact tracing remains a key issue for local authorities, who have only achieved a 55% coverage rate, the ministry said.

Officials also are yet to identify the outbreak’s patient zero and still need to trace more than 35,000 people who have come in contact with infected individuals as of last week, authorities said.

That’s partly because eastern Congo is also battling ongoing violence from rebels. In Ituri, attacks by the Islamic State group-backed Allied Democratic Force have cut off access to many villages and forced people to flee their homes, including those sheltering in overcrowded camps and others constantly on the move.

More than a month into the outbreak, officials believe the disease continues to outpace response efforts and no one knows its true scale.

“If you want to control an outbreak, especially Ebola outbreak, you must know the index case. We don’t have confidence on when this outbreak started,” the Africa Centers for Disease Control and Prevention Director-General Dr. Jean Kaseya told The Associated Press last week.