Congress in recent years has pumped billions of dollars into health preparedness to handle infectious disease outbreaks like Ebola, but the current outbreak in an unstable part of Africa is posing problems that might not be solved by investments alone.

Top Trump administration health officials on Thursday told the Senate appropriations panel overseeing discretionary health funding that the biggest challenges to controlling the Ebola outbreak underway in the Democratic Republic of Congo are a lack of security and a lack of trust for health care workers and government within the local population.

Tim Ziemer, who leads the bureau for democracy, conflict and humanitarian assistance at the U.S. Agency for International Development, told members of the Senate Appropriations Labor-HHS-Education Subcommittee that those were more difficult problems to solve than Ebola itself.

“Security and the community distrust are the two major vectors that seem to be going in the wrong direction,” he said. “We understand the virus.”

The outbreak was declared in August 2018 and is now the largest since the 2014 outbreak that spread in other West African countries. As of March 12, there were 584 deaths and 927 total cases, according to the World Health Organization.

The response is being led by the Democratic Republic of Congo’s health ministry and the World Health Organization.

Health facilities have faced vandalism or looting, especially during unrest following the postponement of elections in some parts of the country in December. Two more attacks on health facilities prompted responders in February to pull out, Ziemer said.

Robert Redfield, director of the Centers for Disease Control and Prevention, told the panel that there is a leadership shortage among health care workers on the ground, and his agency’s most seasoned Ebola experts can’t be on the front lines.

That’s because the security personnel at the U.S. embassy in the country have final say over whether it’s safe for health officials to be in the country’s outbreak zones. The State Department didn’t respond to a request for comment.

For now, Redfield said the agency is still engaged with the outbreak. Since the outbreak began, 130 people deployed to DRC, its surrounding countries and at WHO headquarters in Geneva. But only five staffers are currently in the capital, Kinshasa, providing technical assistance to the country’s health ministry.

CDC Ebola experts who were in Beni, on the country’s northeastern border with Uganda, were removed due to security concerns.

“CDC is prepared to be in the outbreak as soon as our security people tell us that we have permission to go there,” he said. He later added: “We’re well engaged. We’re just not engaged where I think we could also be of critical help to begin a path to bringing this to an end.”

But even if CDC personnel can get closer to the outbreak, they would have to overcome local distrust of government and health care services.

Almost half of the infected people are dead before health officials are aware of them because patients “don’t trust the system to come and present themselves while they’re sick,” Redfield said. “It’s a disastrous indicator for an Ebola response, because people get symptomatic, they get infectious, and if they don’t present themselves, then that’s why we have an ongoing epidemic.”

To be sure, even if the security situation has limited the CDC’s ability to respond, the U.S. government has been able to contribute in other ways. An experimental Ebola vaccine, which grew from a partnership between the National Institutes of Health and the Liberian government during the 2014 outbreak, is being used in a vaccination campaign.

Experimental Ebola treatment drugs are also being tested in controlled trials in the DRC, which are a partnership between that country’s health research agency and the NIH’s National Institute of Allergy and Infectious Diseases, the institute’s leader, Anthony Fauci, told the subcommittee.

“Hopefully by the end of this epidemic, we’ll know which of these work,” Fauci said.