A Senate leader said Sunday she thinks terrorist sleeper cells are in the U.S. — a claim sure to spark even greater fears of a strike on the homeland as the White House launches new efforts to confront growing Islamist extremism around the globe.

Sen. Dianne Feinstein, California Democrat and her party’s ranking member on the Senate Select Committee on Intelligence, flatly stated that she believes terrorist cells are hiding in Europe and the U.S., waiting to be activated and carry out attacks similar to the ones that claimed 17 lives in France last week.

“I think there are sleeper cells not only in France but certainly in other countries and, yes, in our own,” she told CNN. “This calls for vigilance. … Hopefully, we can be more active in terms of doing those things which enable us to find terrorists, see who they’re communicating with and to track that.”

Mrs. Feinstein and Obama administration officials did not elaborate on how many suspected terrorists are in the country, nor did they discuss details of any terrorist plot.

Despite the apparent lack of specific threats, polling data show the American people are deeply worried about a strike in the U.S.

A Rasmussen poll released Sunday shows that 65 percent of Americans believe it is at least somewhat likely that an attack “on those critical of Islam” in the U.S. will occur over the next year. Just 26 percent said such an attack is not likely, the survey shows.

The poll was released just days after a string of terrorist attacks shook France. The violence began Wednesday with a mass shooting at the satirical newspaper Charlie Hebdo, where 12 people were killed.

At least 1 million people — including more than 40 world leaders such as German Chancellor Angela Merkel and Israeli Prime Minister Benjamin Netanyahu — marched in Paris on Sunday to honor the victims of last week’s attacks and to rally for unity and freedom of expression.

The U.S. sent only its ambassador to France, Jane Hartley. Even Russia sent a higher-ranking official to the rally in Foreign Minister Sergey Lavrov.

“Today, Paris is the capital of the world,” said French President Francois Hollande. “Our entire country will rise up toward something better.”

Outgoing U.S. Attorney General Eric H. Holder Jr. was among those in Paris on Sunday, though he was not seen at the march, according to media reports. He appeared on at least four Sunday morning political television shows in an attempt to reassure the American public that the administration is doing all it can to counter extremism.

The White House also announced that Mr. Obama would host an anti-extremism summit Feb. 18. The gathering will focus on efforts to stop the radicalization of Americans by terrorist groups such as al Qaeda and the Islamic State, and ways to combat terrorist recruiting efforts.

In the meantime, Mr. Holder said the federal government is keeping a close eye on individuals who may be part of the sleeper cells Mrs. Feinstein is warning about.

“We are constantly evaluating where we stand with regards to those people who we have suspicions about. We have a universe of people who we focus on using legitimate means and keep track of them,” Mr. Holder said on NBC’s “Meet the Press.” “The American people should feel secure in what it is we do. … We are doing, as I said, the best we can.”

But lawmakers believe more must be done in the wake of the attacks in France, which started when brothers Said and Cherif Kouachi stormed the Charlie Hebdo offices. The men targeted the paper because it published cartoons mocking Muhammad, Islam’s prophet. Al Qaeda’s branch in Yemen said it directed the attack.

A day later, another terrorist, Amedy Coulibaly, killed a police officer and then took hostages inside a kosher market before police raided the building. Over the weekend, video emerged of Coulibaly pledging allegiance to the Islamic State.

The Kouachi brothers reportedly had weapons training in Yemen and were still able to travel back to France and carry out attacks. American lawmakers fear foreign fighters, after receiving training in Yemen or elsewhere, could travel to the U.S. and unleash violence.

“We’ve had thousands of these Western travelers that have become foreign fighters in Iraq, Syria and Yemen that pose a threat and a risk of returning to where they came from and to do the same thing we saw in Paris,” Rep. Michael T. McCaul, Texas Republican and chairman of the House Homeland Security Committee, said during a Sunday appearance on “Face the Nation” on CBS.

“We don’t have a good handle intelligencewise of who is on the ground in Syria, Iraq and Yemen to identify them and put them on no-fly lists so they can’t get into the country,” he said. “Europe has to strengthen and tighten its travel restrictions. We need to look at protecting this country.”

Both Mr. McCaul and Mrs. Feinstein singled out the U.S. visa waiver program as a potential problem area. The program allows citizens from some countries to enter the U.S. without a visa.

Mrs. Feinstein called it “the Achilles’ heel of America” and said it must be reformed.

“They can come back from training, they go through a visa waiver country and they come into this country,” she said. “We have a big problem here.”

Moving forward, Mr. McCaul said, the House Homeland Security Committee will launch an investigation into potential security gaps in the visa waiver program or other areas that may allow foreign fighters to make their way to the U.S.

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