A labeling issue is to blame for the accidental release of a coronavirus patient who was on her way back to Marine Corps Air Station Miramar to join other quarantined evacuees when officials found out her test was positive, the Centers for Disease Control and Prevention (CDC) said.

The woman, who has not been identified, was one of four individuals who was transported from the base to UC San Diego Medical Center for observation after developing possible symptoms of coronavirus. Initially, officials said that all four individuals tested negative for the illness, however, the woman’s sample wasn’t included in the initial test batch.

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“At the time, they were relying on phone conversations to relay test results and the CDC in Atlanta told officials in San Diego that all tests were negative,” a CDC representative said on Tuesday. “The problem was because of differences between San Diego labeling and what the CDC lab accepts, the sample from the woman didn’t even reach testing stage so it was true that all samples that were tested were negative, and it was also true that the woman had COVID-19 [coronavirus] because her sample was not tested.”

The official said the hospital and CDC have since discussed protocol in “excruciating detail” to ensure that the same mishap doesn’t happen again.

UC San Diego Medical Center officials said on Tuesday that the pseudonyms they used on the test labels to protect patient privacy did not match the ones used by the CDC.

“It was an issue with labeling,” Dr. Christopher Braden, the CDC delegate in San Diego, said in a news conference. “They have their procedures, we have our procedures. They didn’t match exactly.”

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Officials received a text message while transporting the four patients back to base that the woman actually tested positive for the virus. The three other passengers in the van who were traveling with her are now considered to be at medium risk and are being kept in isolation at Miramar.

The woman was sent back to the hospital.

Braden said the woman was not symptomatic while on the plane from China to Miramar, and that the other passengers in the van were all wearing masks. He said they will continue to be monitored for several days, but that it doesn’t affect the other evacuees on base.

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The woman’s positive result marks the 13th case confirmed in the U.S. Three of the cases have since been released from the hospital to their homes, including a man in Washington and a couple in Chicago.

On Monday, officials released the first 195 evacuees who had arrived in the U.S. from Wuhan, the virus epicenter, after they completed their 14-day quarantine at March Air Reserve Base in Riverside County, California.

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The virus has sickened over 45,000 globally, and in China, there have been more than 1,100 deaths. Most of the fatalities have occurred in the Hubei Province, which includes Wuhan. An additional death was confirmed in Hong Kong, and another in the Philippines.

The Associated Press contributed to this report.