Michael Mcgarrity, assistant director of the Counter-terrorism Division, revealed an average of two suspected terrorists are arrested every week in the US.
He compared it to the fairground favourite, where players have to bonk fake moles as soon as they pop their heads up out of the board.
The top cop chillingly revealed that a jihadi can attack in just 30 days once radicalised.
He said every FBI bureau in all 50 states is working on terror investigations – with up to 3,000 currently under way across the US.
Extremists are increasingly becoming radicalised in isolation.
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And he warned there was a “lack of resources” to fight the tide of terrorism in the US.
Speaking at the Security & Counter Terror Expo in London, he said: “It’s a high-op tempo – some call it whack-a-mole.
“The reality is radicalisation time can be short to long, and mobilisation time is what concerns us.”
The FBI chief added the main threat to the US are “homegrown violent extremists (HVEs)”, saying “the ocean doesn’t matter anymore”.
HVEs are usually born in the US and are aged between 19 and 25.
He said the the “lone wolf behaviour” of “people living in the basements of their parents’ house” makes them harder to detect for the FBI.
Social media and the internet is also being used to feed their radicalisation, with the age of wannabe terrorists slowly creeping down into the teens.
Mr Mcgarrity, who has been in the job only three weeks, was on the scene when suspected jihadi Sayfullo Saipov ploughed a truck down a bike path in New York, killing eight.
He was also on the scene of 9/11 attack on the World Trade Center back in 2001.
Complex plots have given way to simplistic lone wolf attacks “straight out of the ISIS playbook”, the FBI boss said.
He described how “rejection” often triggers wannabe terrorists, with them desperate to be “part of something bigger than themselves”.
Terrorist threats remain a major threat across the world, despite ISIS facing a military defeat the Middle East.
Met Police chief Mark Rowley warned social media is allowing terrorists to plot attacks “with impunity” in the UK.