The U.S. Navy has announced that 13 of its sailors from the USS Roosevelt have now retested positive for coronavirus. After catching the virus and recovering from it, the sailors had tested negative twice.

When they retested positive, the Navy confirmed that they had been practicing social distancing.

They “met rigorous criteria, exceeding CDC guidelines,” the Navy said. However, even after testing negative twice, they started showing influenza-like symptoms.

Following the outbreak, the sailors have been docked in the Micronesian state of Guam. None are currently aboard and they are all under medical surveillance on Naval Base Guam, a Navy spokesperson reported.

The 13 men are not the only ones to have retested positive for COVID-19. South Korea has also reported a growing number of people who got twice infected. Scientists are now investigating whether the virus can become dormant and resurface after a while.

Stanley Perlman, a professor of microbiology and immunology at the University of Iowa, told NPR that it is still uncertain whether those who recover have gained complete immunity.

“We just really don’t know. It’s even hard to speculate,” the professor said.

Earlier in April, one sailor from the USS Roosevelt died from coronavirus and Capt. Brett Cozier was fired under the allegation of mishandling the outbreak. Others, instead, were just hospitalized.

Navy Secretary James McPherson requested a more thorough investigation of the event to reappraise Cozier’s role in the incident.

CORONAVIRUS FAQ: WIKI OF MOST FREQUENTLY ASKED QUESTIONS