The J. Edgar Hoover Building in Washington is a threat to national security. It’s time to tear it down.

The Federal Bureau of Investigation’s headquarters has been falling apart for years. At this point its decrepitude imperils the bureau’s biggest mission: to prevent another Sept. 11 attack.

If you’ve ever been in Washington, you know the place. You can’t miss it. It’s that pile of concrete defacing Pennsylvania Avenue, halfway between the White House and the Capitol, that looks like a Stalinist parking garage, an architectural crime against humanity. It’s a public disgrace.

The building’s ugliness is a perfect match for its shoddy construction. Rainwater pours into the building, and its thick skin is cracking. Passers-by run the risk of being brained by a falling chunk of FBI facade.

On the inside, things are worse. Never mind that the electrical systems, the air ducts and the elevators need $80 million worth of repairs. The Hoover building fails to meet basic security criteria established for federal buildings three years ago.

These failures are, of course, classified. But they are no secret: The FBI headquarters can’t support its fraying patchwork of computer systems or secure its cubicles from cyberspies.

What’s more, the building is as crowded as a rush-hour subway — and as incompatible to candid conversations among agents. FBI counterterrorism specialists need to be able to speak with FBI intelligence analysts face-to-face, in private. But the Hoover building’s space constraints prevent them from sitting down together in one place.

Instead, they are closed behind an endless series of doors, unable to adequately collaborate. That might have been fine when Hoover ruled as an absolute monarch, hurling commands from on high. It’s a bad business in this day and age, when intelligence and analysis need to flow up the chain of command.

The place already was overflowing on the day its present director, Robert Mueller, took office on Sept. 4, 2001. Back then the FBI had 9,700 people assigned to headquarters. Today it has 17,300. Half of them can’t fit into the building. After Sept. 11, 200,000 square feet of basement, storage and cafeteria space were converted to offices. When that ran out, FBI administrators started renting space in suburban shopping malls and corporate parks. Rent for these remote offices now exceeds $3 million a week.

More than 8,000 FBI headquarters personnel are housed at more than 40 annexes inside and outside Washington. Landlords and tenants in at least nine of these places refuse to operate under the government’s security constraints. So the offices are useless for anything resembling counterterrorism.

On the contrary, if you want to see what a palace built with tax dollars looks like, go to the Central Intelligence Agency’s headquarters complex. (No, they don’t have guided tours; neither does the FBI since al Qaida struck.) The main building, dating from President Dwight D. Eisenhower’s administration, has a gleaming marble lobby and snazzy, Mad Men- style executive suites on the seventh floor. The newer buildings, added under Presidents Ronald Reagan and George H.W. Bush, are state-of-the-art — good-looking and girded against hackers, from the wires to the windows.