Acting Navy Secretary James McPherson said he still had questions about the case after last week's preliminary report to Secretary of Defense Mark Esper.

The decision left undecided the fate of the captain of the USS Theodore Roosevelt, Brett Crozier, who was removed on April 2.

Crozier allegedly broke his chain of command to make public the outbreak of dozens of cases of COVID-19 among the warship's crew. The caseload has risen beyond 900 since he spoke out.

"I have unanswered questions that the preliminary inquiry has identified and that can only be answered by a deeper review," McPherson said in a statement.

He said the probe would build on the inquiry to provide a "more fulsome understanding of the sequence of events, actions, and decisions of the chain of command."

Crozier was removed by then-acting Navy secretary Thomas Modly after writing an unclassified letter to his superiors -- which leaked to the media -- spelling out the threat an initial handful of shipboard COVID-19 infections posed to the crew of almost 5,000.

Modly had been frustrated by lack of support for his proposal to evacuate the ship, which had docked in Guam on March 27, to stem the outbreak.

"The spread of the disease is ongoing and accelerating," Crozier wrote in a March 30 letter. "We are not at war. Sailors do not need to die."

Modly himself resigned days later, after a ranting, profanity-laced speech to the Roosevelt crew in which he accused Crozier of "betrayal."

Since Crozier's warning about the outbreak, the Roosevelt has been largely evacuated for sterilization, and 969 sailors, one-fifth of the crew, have tested positive for COVID-19.

The case has raised criticism from Congress that Esper, following President Donald Trump, did not move quickly enough when the COVID-19 threat emerged. The Pentagon has rejected those charges.

On Wednesday the Navy said the sterilization of the Roosevelt had been completed and it was now being readied for return to sea.

Meanwhile on Tuesday a second Navy ship struck by a coronavirus outbreak, the guided missile destroyer USS Kidd, sailed into San Diego for evacuation.

The Navy said at least 64 out of the 300-strong crew had tested positive for the virus.