The US Navy announced that five sailors who had returned to the nuclear-powered aircraft carrier USS Theodore Roosevelt after a coronavirus quarantine, have tested positive for the virus again.

“This week, five USS Theodore Roosevelt sailors who previously tested COVID-19 positive and met rigorous recovery criteria, exceeding CDC (Centers for Disease Control and Prevention) guidelines, have re-tested positive,” the US Navy said in a statement on Friday.

The sailors, the statement said, developed flu-like symptoms and “did the right thing reporting to medical for evaluation”, reports Xinhua news agency

“The sailors were immediately removed from the ship and placed back in isolation, their close contacts were mapped, and they are receiving the required medical care,” it added.

“A small number of other sailors who came in close contact with these individuals were also removed from the ship and tested. They will remain in quarantine pending retest results.”

In a Pentagon briefing on Friday, chief spokesman Jonathan Hoffman called the outbreak on USS Theodore Roosevelt a “learning process”.

“It’s a stubborn disease. We’re learning a lot. We’re taking every step possible to get the ship fully clean and fully ready,” he said.

The outbreak on the aircraft carrier, in which more than 1,000 sailors were reported to have infected with the coronavirus, came under spotlight after Brett Crozier, then commander of the ship, wrote a letter in March pleading for help evacuating the ship, when dozens on board had tested positive for the virus.

In doing so, Crozier was relieved of his command by then-acting Secretary of the US Navy Thomas Modly, who himself resigned later after his remarks lashing out at the captain backfired.

More than 4,000 sailors from USS Theodore Roosevelt’s nearly 5,000-person crew were taken off the ship to quarantine after arriving in Guam on March 27, according to the US Navy.

In late April, the US Navy began returning sailors to the aircraft carrier.

Returning to the ship required testing negative two consecutive times.

The US Navy has been conducting an investigation into the situation on USS Theodore Roosevelt.

A Pentagon official has also announced that it would also investigate the Navy’s response to the pandemic on its ships.