It happened again: The FBI had the future Chelsea bomber on its radar — for a while, anyway — but let him slip through. Just as officials had done with men who became the perps in at least eight other terror attacks.

Customs officials screened Ahmad Rahami in 2011 on his return from a trip to Pakistan, and again after a 2013-2014 trip there. They notified Homeland Security Department’s National Targeting Center — which sent the report on to the FBI and other agencies.

Then, in August 2014, Rahami’s dad told the authorities his son was a terrorist. Agents checked records and found nothing. When his father recanted, they say, they closed the book. Yet they never bothered to interview Ahmad Rahami himself.

The dad this week made it sound like he rang the alarm pretty hard, but he may have ulterior motives: Paul Sperry reports in today’s Post that other Rahami family members at the very least had serious warnings Ahmad was turning toward terror.

Then, too, the feds have limited resources. As John Miller, the NYPD deputy commissioner for counterterrorism, notes, “People have somewhat of a misconception about our ability to put someone under surveillance [and] leave them there indefinitely.”

FBI Director James Comey notes that searching for lone wolves is like “looking for needles in a national haystack.” But Rahami was less a lone wolf than what Pat Poole at PJ Media calls a “known wolf” — i.e., someone who had been flagged by authorities but then forgotten.

Poole cites at least eight other such “known wolves” — including the Underwear Bomber, the Fort Hood shooter and perps in the Orlando nightclub massacre and Boston Marathon bombing as well as jihadis in Garland, Texas; Little Rock, Ark.; Seattle; West Orange, NJ; and Columbus, Ohio.

A lack of resources doesn’t explain all these dropped balls. In the Fort Hood case, officials ignored an Army officer’s e-mails with terror-preaching imam Anwar al-Awlaki. It sure looked like a bureaucratic mindset took control — putting security concerns a distant third behind the fear of being accused of Islamophobia or of violating someone’s civil rights.

Bureaucratic inertia shows up at the other extreme of anti-terror efforts as well — in genuine, and maddening, intrusions on civil liberties to no sane purpose. The Transportation Security Administration, most notably, has no qualms about putting infants and old ladies through hell at airports.

Your name can land on the terror watch list or no-fly list at the drop of a hat, and you won’t even be told. And good luck trying to appeal — the process for getting off is straight out of Kafka.

Yes, hindsight is 20/20, and the feds have thwarted dozens of plots that no one ever hears of. But we think Americans would feel a lot more safe if the vast federal security apparatus operated more like the NYPD and less like a bureaucracy.