It’s just the sort of scenario that worries Washington policymakers at the highest levels — the threat of impressionable homegrown youths susceptible to the online advocacy of al Qaeda.

And that very scenario was described in detail this week in Brooklyn federal court, where US-born Brooklynite Betim Kaziu stands trial for allegedly plotting to kill Americans abroad after becoming indoctrinated in jihadist ways — largely online — along with his childhood pal Sulejmah Hadzovic.

Now Hadzovic is the chief government witness against Kaziu. Yesterday, he described how they traveled to Egypt and engaged in serious discussions with fellow aspiring jihadists about dying a martyr.