On Friday, visitors to those sites were met with a message that begins: “This domain name has been seized by the F.B.I.” The government used the same controversial tactic of seizing domain names in actions last year against sites accused of copyright violations. Losing a domain name can be costly for a company that has invested in promoting it as the main route to its site. But the tactic can also be of only temporary effectiveness, because the company can switch to a new Web address that is outside the reach of law enforcement in the United States — one that, for example, does not end in .com.