THE HILL: Senator: Have to assume sleeper cells in US.

A key U.S. senator said Sunday that Americans have to assume there are some “sleeper cells” within the U.S. that would like to carry out a terror attack.

Sen. Ron Johnson (R-Wis.) said he was not aware of any specific cells, but noted that the recent attacks elsewhere in the Western world make it a safe assumption. Johnson is chairman of the Senate Homeland Security and Government Affairs Committee.

“What you’re seeing happen in Europe, you’re seeing how widespread that is,” he said on “Fox News Sunday.” “I think you have to assume that that is a risk that we have to consider.”

Johnson criticized President Obama’s policy to combat terror groups, arguing the administration is insufficiently committed to the cause to battle against the Islamic State in Iraq and Syria (ISIS).

“As long as ISIS is not losing, they are perceived as winning,” he said. “We have to defeat ISIS. We can’t let them continue threatening the rest of the world for years.”

He also criticized administration efforts to shut down the detention center at Guantanamo Bay, warning that remaining captives pose a major threat to the U.S.

“The people left in Guantanamo Bay are evil people,” he said. “The people there are the worst of the worst, and they should not be released.”

Sen. Ben Cardin (D-Md.), a member of the Foreign Relations Committee, said U.S. intelligence needs to focus on the greatest threats to home.

“In some cases these are Americans that we know have been associated with these extreme groups,” he said. “It’s really important that our intelligence community focus on those that are the highest risks to the United States.”

Related: Leon Panetta: We face ‘more aggressive’ terrorism.