An explosion damaged a Japanese oil tanker as it exited the Persian Gulf on Wednesday. Though the cause of the blast was not immediately known, the ship’s owner said the vessel may have been attacked.

If an attack, it would be a rare assault on a tanker in the Gulf or at the Strait of Hormuz, a transit point for about 40 percent of oil shipped by tankers worldwide. Al—Qaeda has in the past carried out attacks on oil infrastructure on land in nearby Saudi Arabia, as well as a 2002 suicide bombing against a French oil tanker off the coast of Yemen.

The blast onboard the M. Star supertanker happened shortly after midnight as it entered the strait, heading out of the Gulf, Japanese shipping company Mitsui O.S.K. Lines said.

Mitsui said the explosion at the back of the ship was believed to be caused by “a suspected attack from the outside” while the ship was passing through Omani waters in the western part of the strategically vital strait, a narrow chokepoint between Oman and Iran at the Gulf’s mouth.

“We believe it’s highly likely an attack,” Mitsui spokeswoman Eiko Mizuno said. “There is nothing that can explode in that part of the vessel.”

One of the ship’s 31 crew members noticed a flash of light right before the explosion, she said, suggesting something may have struck the vessel. The explosion occurred at the back of the tanker, near an area where rescue boats are stored, causing cuts to a crew member who was struck with broken glass.

Yuki Shimoda, an official at Japan’s Ministry of Land, Infrastructure, Transport and Tourism said the ministry was not immediately suspecting an attack, but added that the possibility cannot be ruled out.

The tanker, loaded with 270,000 tons of oil, was heading from Das island in the United Arab Emirates to the Japanese port of Chiba outside Tokyo, the ministry said. It said the tanker is registered in the Marshall Islands.

It was not immediately clear what caused the blast.

The Strait of Hormuz is a vital shipping lane for crude oil and other goods headed out of the Persian Gulf. It is far from areas where Somali pirates typically prey on slow—moving ships, though smugglers are known to operate in the area between Iran and an enclave of Oman on the other side of the strait.

The Japanese ministry said none of its ships has been attacked by pirates in the area.

Omani officials couldn’t immediately be reached. Officials in the UAE, whose waters ships cross on both sides of the strait, said they had no immediate information.

Iran has in the past threatened to close the strait if the United States attacks it over Tehran’s disputed nuclear program, though there were no immediately signs of Iranian involvement.

The U.S. Navy’s Bahrain—based 5th Fleet, which patrols the region, said it is investigating the explosion but does not know what caused it.

“We’ve heard about it. We’re still in the process of trying to get details,” said Commander Amy Derrick—Frost.

Initial reports from the ship’s owner say one life boat was blown off the ship, and some starboard hatches were damages, according to the Navy. It said it offered to assist the tanker after the explosion but was told no help was needed.

After the blast, the tanker was headed to the Emirati port of Fujairah under its own power.

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