A key figure behind the terrorist attacks in the United States on Sept. 11, 2001, came to Japan around 1987 as a company trainee, Japanese security sources said Monday.

Khalid Shaikh Mohammed, believed to be the al-Qaeda terrorist network's No. 3 man, was trained as a rock drill operator by a construction machinery maker in Shizuoka Prefecture during his three-month stay, the sources said.

He was arrested in Pakistan in March.

Mohammed is believed to have been involved at the time in Afghan guerrilla activities against the Soviet Union and is suspected of using his Japanese training to dig cave complexes.

Leaving Japan with about 150 rock-drilling machines bought with unknown financial resources, Mohammed began a repair shop for the machinery in Afghanistan, the sources said.

Born in Kuwait around 1965, he joined the group led by Osama bin Laden at its beginning.

When Mohammed was receiving training in Japan, Washington was aiding Afghan guerrillas. Its relationship with al-Qaeda, set up in 1988, turned hostile in 1990.

The U.S. blames al-Qaeda for the terrorist attacks on the World Trade Center and the Pentagon that killed around 3,000 people.