Emirates' WAM news agency on Friday quoted an unnamed government official as saying the investigation revealed traces of homemade explosives on the hull of the tanker.
WAM said investigators believe a small boat carrying explosives had approached the tanker and that the presence of explosives "indicates the tanker was subjected to a terror attack".
Seiji Maehara, the Japanese Transport Minister, has instructed his ministry to ask the UAE to check the facts of the investigation, Kyodo news agency reported.
There have been conflicting reports about what happened to the M. Star supertanker in the Strait of Hormuz, a transit point for about 40 per cent of tanker-shipped oil worldwide.
A crew member was injured and the vessel sustained a square-shaped dent on the rear side of the hull during the incident.
The Marshall Islands-flagged ship, loaded with 270,000 tons of oil, was heading from the petroleum port of Das Island in the United Arab Emirates to the Japanese port of Chiba outside Tokyo.
In its claim posted on the internet, the al-Qaeda-linked Brigades of Abdullah Azzam carried a photo of the purported bomber pointing to a photograph of a tanker on a laptop.
It said the bomber was a "martyr'', meaning he had died in the attack.
The group also said it had delayed the announcement of the incident until several group members who were involved in the operation "returned safely to base".
The WAM report said the vessel left the Emirati port of Fujairah on Friday after the damage to the hull was fixed.
The Brigades have in the past claimed responsibility for the August 2005 firing of Katyusha rockets that narrowly missed a US amphibious assault ship docked at Jordan's Aqaba Red Sea resort, killing a Jordanian soldier.
It had also claimed the July and October 2004 bombings at Egypt's Red Sea resort of Sharm el-Sheik and two other resorts that killed a total of 98 people.
Al-Qaeda has 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 and the 2000 bombing of the USS Cole in the Yemeni port of Aden.