WASHINGTON — Investigators in the United Arab Emirates said Friday that a terrorist attack caused the mysterious damage a Japanese oil tanker suffered last week as it passed through the Strait of Hormuz, raising fears of future attacks in the narrow channel that serves as a passageway for shipping crude oil from the Middle East to the rest of the world.

The damage to the tanker — which an Emirati official said was caused by “homemade explosives” aboard a dinghy — was not considered serious, and there was little immediate impact on oil markets on Friday.

But the news instantly fanned worries about shipping security. If confirmed, the attack would be the first of its kind in the volatile strait, which has long been a focal point for tensions with Iran, just across the water from the Arabian Peninsula.

About 17 million barrels of oil a day pass through the strait, close to 40 percent of the oil shipped by tankers worldwide.

The account of the attack came in a report published Friday by the state-run Emirates news agency WAM, from an Emirati coast guard official.

Earlier this week, the Abdullah Azzam Brigades, a militant group with ties to Al Qaeda, claimed it had carried out a suicide attack against the tanker, the M. Star.

American officials on Friday would not confirm that the episode was a terror attack, but one intelligence official said that the damage to the tanker — a large square dent on the hull’s port side — was “from an external explosion.” The official said it remained unclear whether the group taking credit for bombing the tanker was indeed involved.

Lt. John Fage, a spokesman for the Navy’s Fifth Fleet in Bahrain, said that a team of Navy divers had recently traveled to Fujairah to assess the ship’s damage.

The Japanese government was conducting its own investigation. Japan’s transportation minister, Seiji Maehara, said at a news conference in Tokyo that the government had obtained samples taken from the damaged portion of the tanker and would “conduct analyses for all possible causes, without prejudice.”

The ship, loaded with two million barrels of oil, was heading toward Japan on July 28 when a sudden force shattered windows, ripped off deck railings and blew off a lifeboat, in addition to punching the dent into its black-and-red hull. One crew member was slightly hurt.

The ship’s owner, Mitsui O.S.K. Lines, said in a statement that it was still investigating the extent and the cause of the ship’s damage. Last week, Mitsui officials said they suspected an attack. Crew members aboard the ship said they had seen a flash and heard an explosion, officials said, but there were no reports of a boat approaching the tanker.

Maritime security experts and port officials in the Middle East had raised the possibility that the tanker had collided with a submarine, hit a degraded sea mine — perhaps left over from the Iran-Iraq war of the 1980s — or had even been hit by a “freak wave” — a theory quickly discarded once investigators began to examine the damage to the ship.

Last week, American military officials and security experts were dubious about the possibility the ship had been attacked, suggesting that a collision of some kind was more likely.

Despite the seemingly amateurish nature of the alleged attack, its implications are serious, analysts said.

“Before, the Iranian naval threat was seen as key,” said Mustafa Alani, a security analyst at the Gulf Research Institute in Dubai. “Now, we have the possibility of Qaeda-type groups that appear to be learning from the tactics of pirates.”

One crucial question, Mr. Alani added, was where the presumed attacker would have come from. The coasts in the region are closely watched.

Recently, jihadist Web sites have seen a number of exchanges about attacking tankers in the Persian Gulf to disrupt international oil markets, said Evan F. Kohlmann, a terrorism analyst at Flashpoint Global Partners, a New York security consulting firm.

The Abdullah Azzam Brigades has been known mostly for its claims to have launched missiles at Israel from southern Lebanon, Mr. Kohlmann said. In its claim of responsibility for the tanker attack, the group said it had aimed at wreaking havoc in international markets.

In that sense, the attack failed, analysts said, coming at a time when OPEC producers have large amounts of spare capacity, global oil inventories are high and oil demand remains sluggish. Oil futures barely reacted to the news, with prices actually falling on Friday in New York by about 1 percent to $81.20 a barrel.

“Back in 2008, there might have been a 50-cent blip, but today it is not even making headlines in terms of its impact on the oil market,” said Greg Priddy, an energy analyst at Eurasia Group, a political risk consulting firm. “It is not going to impede the tanker business or the oil market in any significant way.”

The Abdullah Azzam Brigades is unusual among jihadist groups in that it operates as a “patchwork of free-floating militant cells” affiliated with Al Qaeda but not directly controlled by it.

The only previous maritime terrorist attacks in the region have taken place on the coast of Yemen, on the southern side of the Arabian Peninsula. In 2000, an Al Qaeda suicide bomber rammed the American destroyer Cole in the port of Aden, killing 17 American sailors. In 2002, a French tanker, the Limburg, was attacked in a similar way a few miles off the coast of Aden, and one crewman was killed.

While tankers are slow and hard to maneuver, making them easy targets, attacks against tankers have been extremely rare. Modern tankers typically have twin hulls, many separate compartments to store oil, and are sturdy enough to withstand a direct hit from rocket-propelled grenades, for instance.

Aside from the attacks on ships that occurred during the Iran-Iraq war, the attack on the Limburg is the only other instance analysts cite of a tanker being attacked by terrorists. That attack, carried out by a small fishing boat packed with explosives, was believed to have been the work of Al Qaeda operatives in Yemen, and was the group’s first successful strike against an oil target.

The Limburg was crippled and leaked some oil, but did not sink.

The damage to the M. Star was even less severe. After a week docked in the Port of Fujairah for repairs and an examination, the tanker departed Friday and began returning to Japan, Emirati and Japanese news agencies reported.


Reporting was contributed by Jack Healy and Jad Mouawad from New York, Nada Bakri from Beirut, Lebanon, Hiroko Tabuchi from Tokyo and Mark Mazzetti from Washington.