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TOKYO (Reuters) - The U.S. Navy on Tuesday said on Tuesday it has temporarily relieved, for medical reasons, the commander of a warship involved in a crash with a container vessel in Japanese waters that killed seven American sailors.

The collision between the guided missile destroyer USS Fitzgerald and the Philippine-registered ACX Crystal on June 17 resulted in the greatest loss of life on a U.S. Navy vessel since the USS Cole was bombed by Islamist militants in Yemen’s Aden harbor in 2000.

At least six investigations have been launched, including two U.S. Navy internal hearings and a probe by the United States Coast Guard (USCG).

“Cmdr Bryce Benson, who is recovering from injuries sustained during Fitzgerald’s June 17 collision with the merchant vessel ACX Crystal was relieved temporarily,” the U.S. Navy Seventh Fleet said in a press release.

None of the investigations has apportioned blame for the accident, or explained how an advanced U.S. warship with sophisticated radars and trained lookouts sailing in clear, albeit dark, conditions was struck by a vessel more than three times its size.

In the first detailed account from one of those directly involved in the collision, the cargo ship’s captain, in a report seen by Reuters, said the ACX Crystal signaled the Fitzgerald with flashing lights about 10 minutes before the collision, but that it did not respond or alter course.

The U.S. Navy has said it would not comment until the investigations were complete.

The Fitzgerald is in dry dock at its home port in Yokosuka, Japan. Engineers are undertaking temporary repairs and will assess whether the damaged vessel can sail back to the United States.