A year after a Nigerian man allegedly tried to blow up a Detroit-bound airliner, officials say they have made it easier to add individuals' names to a terrorist watch list and improved the government's ability to thwart an attack in the United States.

The failure to put Umar Farouk Abdulmutallab on the watch list last year renewed concerns that the government's system to screen out potential terrorists was flawed. Even though Abdulmutallab's father had told U.S. officials of his son's radicalization in Yemen, government rules dictated that a single-source tip was insufficient to include a person's name on the watch list.

Since then, senior counterterrorism officials say they have altered their criteria so that a single-source tip, as long as it is deemed credible, can lead to a name being placed on the watch list.

The government's master watch list is one of roughly a dozen lists, or databases, used by counterterrorism officials. Officials have periodically adjusted the criteria used to maintain it.

But civil liberties groups argue that the government's new criteria, which went into effect over the summer, have made it even more likely that individuals who pose no threat will be swept up in the nation's security apparatus, leading to potential violations of their privacy and making it difficult for them to travel.

"They are secret lists with no way for people to petition to get off or even to know if they're on," said Chris Calabrese, legislative counsel for the American Civil Liberties Union.

440,000 on list



Officials insist they have been vigilant about keeping law-abiding people off the master list. The new criteria have led to only modest growth in the list, which stands at 440,000 people, about 5 percent larger than last year. The vast majority are non-U.S. citizens.

"Despite the challenges we face, we have made significant improvements," Michael E. Leiter, director of the National Counterterrorism Center, said in a speech this month at the Center for Strategic and International Studies. "And the result of that is, in my view, that the threat of that most severe, most complicated attack is significantly lower today than it was in 2001."

The master watch list is used to screen people seeking to obtain a visa, cross a U.S. border, or board an airliner in or destined for the United States.

The standard for inclusion on it remains the same as it was before - that a person is "reasonably suspected" to be engaged in terrorism-related activity. But another senior counterterrorism official, who like some others would speak only on the condition of anonymity, said that officials have now "effectively in a broad stroke lowered the bar for inclusion."

Timothy Healy, director of the FBI's Terrorist Screening Center, which maintains the master list, said the new guidelines balance the protection of Americans from terrorist threats with the preservation of civil liberties. He said the watch list today is "more accurate, more agile," providing valuable intelligence to a growing number of partners that include state and local police and foreign governments.

Each day there are 50 to 75 instances in which a law enforcement official or government agent stops someone who a check confirms is on the watch list, a senior official at the Terrorist Screening Center said. Such "positive encounters" can take place at airports, land borders or consular offices, or during traffic stops.