America’s vaunted global surveillance is a vital tool for U.S. intelligence services, especially as an early-warning system and as a way to corroborate other evidence. In Crimea, though, U.S. intelligence officials are concluding that Russian planners might have gotten a jump on the West by evading U.S. eavesdropping.

“Even though there was a warning, we didn’t have the information to be able to say exactly what was going to happen,” a senior U.S. official says…

Still, as Russia brings additional forces to areas near the border with eastern Ukraine, America’s spy chiefs are worried that Russian leaders might be able to cloak their next move by shielding more communications from the U.S., according to officials familiar with the matter. “That is the question we’re all asking ourselves,” one top U.S. official says.

The Obama administration is “very nervous,” says a person close to the discussions. “This is uncharted territory.”