Improving the sharing of information between the U.S. and its allies is crucial to stopping
"One nation cannot by itself hope to forestall the possibility of terrorism, even within its own borders," Holder said in an interview from Paris on ABC's "This Week with
Holder also said he was concerned about so-called "lone wolf" attacks by small groups of terrorists, although it's unclear whether the Paris assault was directed by extremist Islamic organizations. One of the gunmen, Said Kouachi, was believed to have trained with militants in Yemen.
"It's something that frankly keeps me up at night worrying about the lone wolf or ... a very small group of people who decide to get arms on their own and do what we saw in France this week," Holder said on CBS' "Face the Nation."
Holder was in the French capital to attend an international security meeting Sunday in the aftermath of Wednesday's attack by terrorists on the satirical newspaper Charlie Hebdo. The attack, and the manhunt that followed, resulted in the deaths of 17 victims.
Hundreds of thousands of people, including world leaders, marched through Paris on Sunday in a show of unity.
Appearing on four Sunday TV talk shows, Holder said the U.S., France and other allies already share information on potential terrorists. But "there's a greater need for us to share information, to knock down these information-sharing barriers, so that we can always stay on top of these threats," he told ABC.
To help with that effort, the White House said it would host an international summit on Feb. 18 "to highlight domestic and international efforts to prevent violent extremists and their supporters from radicalizing, recruiting, or inspiring individuals or groups in the United States and abroad to commit acts of violence."
Gerard Araud, France's ambassador to the U.S., called the summit "a very good idea" and said there needed to be a global strategy to fight terrorism.
"In a sense, France was not attacked as France," he said on ABC. "France was attacked as a western democracy and it could have happened everywhere in the world and unfortunately I guess also in the U.S."
The U.S. did not have any credible information yet on whether terrorist groups were behind the attack, Holder said.
On Saturday, French Prime Minister Manuel Valls declared that his nation was at war with radical Islam. Holder did not go that far.
"I would say that we are at war with the terrorists who commit these heinous acts and who use … a corrupted version of Islam to justify their actions," he said on NBC's "Meet the Press."
"We are bound and determined to hold them accountable, to find them wherever they are," Holder said.
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The threat now comes from Al Qaeda affiliates, such as the Yemen-based Al Qaeda in the Arabian Peninsula, Holder said. That's the group Kouachi is believed to have trained with. U.S. intelligence officials are trying to determine whether the group played a role in last week's attack.
Holder said Al Qaeda affiliates have moved to "smaller kinds of attacks" involving just a few people.
The attack in Paris and recent ones elsewhere around the world have shown "a very small number of people, without huge amounts of planning, without huge amounts of resources, inflicting very severe damage," Holder told ABC.
Senate Armed Services Committee Chairman John McCain (R-Ariz.) said he didn't think the Paris attack was the work of lone terrorists.
"The nature of that attack showed a degree of professionalism that no lone wolf could have carried out," McCain said on CNN's "
He criticized the Obama administration for not doing more to halt terrorist groups operating in Yemen and Syria, where they can train people to attack the U.S. or other nations.
"It's one thing to have a lone wolf, and that's bad. But far worse is a person who has been trained and equipped and sent on a specific mission," McCain said. "That's where you get the big attacks."