Sen. Dianne Feinstein, D-Calif., said that a mass casualty attack orchestrated by terrorists was “within the realm of possibility” during an appearance on CNN’s State of the Union on Sunday.

Feinstein said that there was “no credible, defined threat” but that such an attack is possible.

Feinstein said that in light of the Charlie Hebdo attack, France must have a “unified national effort to rid the country of terrorists – and that’s difficult to do.”

Feinstein said that there are “sleeper cells” that are “deeply embedded” in France and other nations, including the United States. “I think there are sleeper cells not only in France, but certainly in other countries, and yes, even in our own,” she said.