Only a jackass brings a grenade to an airport — and cops say that’s exactly what Johnny Knoxville did today.

Law enforcement sources tell us Knoxville was detained this morning at LAX after security found a fake grenade in his carry-on bag. He was in terminal 4 (American Airlines) when a screener saw something suspicious.

Knoxville was detained, cited and released. We’re told the citation was for carrying a “prohibited item” into an airport — replicas of weapons are prohibited.

But check this — Knoxville made his flight.