In 1914, it was enemy aliens.
In 1930, it was Wobblies.
In 1957, it was fellow travelers.
In 1971, it was insane telepaths.
And, in 1973:
"We don't know what the hell it is," said Andrew J. Burris, Director of the FBI. He threw his hands in the air and looked baffled and confused.
Kenneth J. Malone tried to appear sympathetic. "What what is?" he asked.