U.S. military satellites spied Russian troops amassing within striking distance of Crimea last month. But intelligence analysts were surprised because they hadn't intercepted any telltale communications where Russian leaders, military commanders or soldiers discussed plans to invade.

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. (Follow the latest developments on the crisis in Ukraine.)

"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.