ABU DHABI—Investigators in the United Arab Emirates have concluded that an Omani suicide bomber in an explosives-laden motorboat attacked an oil tanker last week as it traveled through the Strait of Hormuz, according to a person familiar with the investigation.

The U.A.E., in a statement Friday, became the first government to say the mysterious early-morning incident on July 28 was a terrorist attack. The U.A.E. led a weeklong probe of the incident, with help from the U.S. Navy and the Oman and Japanese governments.

U.S. military officials said they couldn't confirm the U.A.E.'s findings, and needed to see more evidence.

Mitsui O.S.K. Lines Ltd., the Japanese company that owns the tanker, has maintained it was hit by an explosion, but officials in the region and U.S. officials have said alternately that an unusually strong wave or collision might have damaged the vessel. The ship was left with a dented hull, and one crewman was injured.

An al Qaeda-affiliated militant group believed to be based in Egypt has claimed responsibility for attacking the tanker, saying it had hoped to disrupt the global oil supply.

The person familiar with the investigation hadn't yet concluded whether the militant group's claim of responsibility was correct. But forensic evidence collected on the ship, including traces of explosives and details collected by intelligence services in the region left no doubt about the nature of the attack, the person said.

"It was a very amateurish attempt, but it was clearly a failed terrorist attack," the person said.

If the assessment is correct, it is thought to be the first time that Islamic extremists have targeted oil shipments in the Strait of Hormuz, through which 20% of the world's global oil supply travels each day.

A spokesman for the U.S. Navy's Fifth Fleet, which is based in Bahrain and patrols the strait, said Navy experts were in contact with Emirati officials to discuss the findings.

The U.S. examination didn't yield evidence of explosives, a U.S. official said, though U.S. divers weren't immediately allowed to examine the tanker after it docked at the U.A.E.'s Port of Fujairah following the incident.

"We don't really know what happened," said a U.S. military official. "We do care a great deal, because of the strategic importance of the Strait of Hormuz."

The ship, the M. Star, left port Friday to continue its journey to Japan, the U.A.E. state-run news agency reported.

A spokeswoman for Japan's Ministry of Foreign Affairs said the government is still investigating the cause of the damage to the ship. A spokeswoman for Mitsui had no comment.

Shipping and oil industry analysts took the latest announcement with relative equanimity, saying they wanted to see more detailed evidence to confirm last week's incident as a terrorist attack.

"Apart from the pictorial evidence of damage to the vessel, there is no corroborated information for insurers to work with," said Neil Roberts, a senior executive of underwriting at Lloyd's Market Association.

Oil markets, focused Friday on weak U.S. jobs data and the implications for weaker crude demand, shrugged off the U.A.E. government's conclusion. Benchmark crude fell $1.31, or 1.60%, to $80.83 a barrel Friday on the New York Mercantile Exchange.

"While this highlights security concerns at the world's most important oil supply choke point, the scale of the attack seems pretty low-brow," J.P. Morgan said Friday in a research note.

The shipping industry has already taken additional security measures and is paying higher insurance coverage because of an increase in Somali pirate attacks over the past couple of years. Some shippers have hired armed private security firms to help protect staff and vessels. Many are altering navigation routes to avoid coastlines, for example, and areas where the threat of piracy is greatest.

American forces are usually at a high rate of readiness in the area because of regional tensions and the strategic importance of the oil traffic that transits the strait. The U.S. hasn't increased its vigilance in the aftermath of the incident.

In a statement posted on a militant website and dated Aug. 2, the Abdullah Azzam Brigades, a group named after one of Osama bin Laden's religious mentors, claimed responsibility for the attack. The group identified the bomber as Ayyub Al Tayshan.

The statement didn't give details about how the group attacked the vessel, or selected that specific ship.

The person close to the investigation said intelligence services in the region have identified the bomber as a young Omani man from a coastal village north of the capital, Muscat. The person said he wasn't authorized to release the bomber's name.

Intelligence agencies in the region have determined that the Omani was in contact with terrorist elements in the region for the past two months, but they haven't conclusively linked him to the Abdullah Azzam Brigades, the person said.

Local intelligence officials don't believe that the bomber had received any formal military or explosives training, the person said. Forensics evidence from the hull of the tanker showed that the explosives used were a homemade compound employing TNT-like material, the person said.

"The bomb he made was homemade and very weak," the person said.

For the past week, Omani intelligence officials have been conducting sweeps along their coast searching for evidence of a wider terrorist network there, according to the person. It is unclear whether the surveillance has yielded any arrests.

—Julian E. Barnes and Spencer Swartz contributed to this article.