Republican senators prodded Homeland Security Secretary Janet Napolitano today to explain how she would implement the pending immigration bill, which gives her numerous powers to tighten or loosen many aspects of immigration law.

“This bill gives you, the secretary, extraordinary discretion in making many decisions about how the [task] would be carried out … that causes me many concerns,” Alabama Republican Sen. Jeff Sessions told Napolitano.

“You’ll have discretion on how much fencing [to build on the U.S.-Mexican border], and the ability to say when it is ‘substantially’ complete,” added Utah Republican Sen. Mike Lee.