A senior member of the al-Qaeda terrorist group may have been in Japan from 2000 to shortly before the Sept. 11, 2001, terrorist attacks in the United States, according to an investigation by Japanese and U.S. public security authorities.

According to sources close to the case, this senior member of the group -- who was not identified -- is believed to have played a role in the attacks.

On the basis of the information, the two countries believe there may still be al-Qaeda sympathizers in Japan and have stepped up cooperation to track the movements of radical Islamic elements here, the sources said Saturday.

The al-Qaeda member in question is believed to have entered Japan sometime in 2000, and was apparently working to raise funds for the group's terrorist activities and attract more supporters, according to the sources. There are also indications that he visited public facilities in Japan, the sources said.

It is highly unlikely that he could have remained in Japan for such a long time without domestic support and the National Police Agency and the Justice Ministry have stepped up cooperation with the FBI and the CIA on the case.