24 Ebola patients slipped out of a treatment center this week in the Democratic Republic of Congo raising fears the virus may spread as health officials raced to trace anyone they may have encountered.

According to the ministry, the patients had to flee when the treatment center came under attack by people protesting the cancellation of Sunday’s presidential election in the eastern city.

Ministry spokeswoman Jessica Ilunga told Reuters that 17 of the patients had already tested negative for Ebola, while seven had not yet been tested.

Ilunga said that three patients had already returned to the center while health workers were in contact with 17 others to coordinate their return.

Health officials have addresses and phone numbers for the remaining four, she added.