The Lebanese man accused of masterminding a plot to destroy Hudson River train tunnels had visited America at least once — a trip to California six years ago, a federal law enforcement official said.

The official,speaking on condition of anonymity because the investigation is ongoing, said 31-year-old Assem Hammoud was in America on a legitimate visa for a brief stay, and that he was believed to have been visiting either family or friends. The visit occurred long before authorities say the tunnel plot began to unfold.

Authorities are still trying to trace Mr. Hammoud’s steps during that trip but say they have no record of him going to New York. They have not ruled out the possibility that Mr. Hammoud had come to the country using different names.

Meanwhile, a senior Lebanese official said authorities there found maps and bombing plans on the personal computer of the suspect. The acting interior minister, Ahmed Fatfat, described the information found on Assem Hammoud’s computer as “very important.”

“It contained maps and bombing plans that were being prepared,” Mr. Fatfat said in a local television interview.

Lebanese security officials said that they obtained “important information” from Mr. Hammoud’s computer and CDs seized from his office at the Lebanese International University, where he taught economics.

“This information helped the investigators make Hammoud confess to his role in plotting a terror act in America,” one Lebanese official said, speaking on condition of anonymity because of the sensitivity of the case.

Mr. Hammoud, who has used the alias Amir Andalousli, has been in Lebanese custody since April. Two others also are in custody in the case, which American investigators said was disrupted after coordinated efforts with officials in six other countries. Five suspects are at large.

The suspects are alleged to have planned to attack PATH trains under the Hudson River using suicide bombers and backpack bombs. The plan, which authorities said the suspects hoped to carry out in October or November, was to flood Lower Manhattan by attacking the tunnels — used by tens of thousands of commuters each day.

But the plot was only in the planning stages, and the suspects had not purchased any explosives or traveled to America as part of the scheme.