SAN FRANCISCO — Four years ago the U.S. Court of Appeals for the Ninth Circuit ruled that an international traveler could be forced to boot up his laptop computer, without any reasonable suspicion, so that files could be inspected at the border.

On Tuesday, an en banc panel sitting in Pasadena wrestled with the next logical step for border-zone searches: whether customs agents can seize a computer and transport it 170 miles for three days of forensic testing on its hard drive.