The federal government's main terrorist watch list has grown to at least 700,000 people, with little scrutiny over how the determinations are made or the impact on those marked with the terrorist label. The government refuses to confirm or deny whether someone is on the list, officially called the Terrorist Screening Database, or divulge the criteria used to make the decisions. Even less is known about the secondary watch lists that are derived from the main one, including the no-fly list (used to prevent people from boarding aircraft), the selectee and expanded selectee lists (used to flag travelers for extra screening at airport checkpoints), the TECS database (used to vet people entering or leaving the United States), the Consular Lookout and Support System (used to screen visa applications) and the known or suspected terrorists list (used by law enforcement in routine police encounters). For people who have landed on these lists, the terrorist designation has been difficult to challenge legally. The Terrorist Screening Center, which administers the main terrorist watch list, declined to discuss its procedures, or to release current data about the number of people on various watch lists, and how many of them are American citizens.