EMBLEMS OF dysfunction in Washington are plentiful, but even in that crowded field, the government’s failure to relocate the FBI from its disintegrating, antiquated downtown headquarters stands out. After years of discussion and tens of millions of dollars expended to find a new site, the federal government on Tuesday pulled the plug on the entire undertaking, in effect saying: Never mind.

What an inexplicable, illogical and irresponsible decision. And what a colossal embarrassment for the United States to waste so much time, effort and money.

In a briefing for stakeholders on Capitol Hill Tuesday, officials of the FBI and General Services Administration, which manages federal real estate, blamed Congress for failing to appropriate sufficient funds to keep the project on track. That’s true; lawmakers have dragged their feet in appropriating most of the estimated $2.5 billion cost.

But there was no acknowledgment of the staggering cost of cancellation, which really only means postponement. After all, the need is undiminished to replace a headquarters that sheds concrete chunks and is badly obsolete. Inflation alone means that every year of delay will add as much as $100 million to the project’s ultimate bill.

Six years ago a government watchdog agency said the FBI had outgrown the J. Edgar Hoover Building on Pennsylvania Avenue, which opened in 1974 — a structure that is “obsolete, inefficient, costly to maintain” and “prohibitively expensive to modernize,” in the FBI’s own view. Five years ago the GSA started putting out feelers for developers to build a new structure or campus. And it was three years ago that the GSA narrowed the potential site options to three suburban locations — two in Maryland’s Prince George’s County, one in Virginia’s Fairfax County.

Now thousands of FBI employees who might have been housed in a single modern structure will instead remain scattered throughout more than a dozen buildings across the metropolitan area, where short-term leases cost taxpayers $55 million annually.

Blame Congress for playing games on appropriating funds for the project, rescinding $200 million in funding last month after having already appropriated less than half the agency’s 2017 budget request of $1.4 billion for the project. Blame the GSA for acting with next to no consultation with Congress, and with unconvincing explanations.

And blame President Trump, irate that the FBI has the temerity to investigate his campaign’s dealings with Russia, for heaping contempt on the agency, fueling suspicion of its motives and undercutting its political and public support. The need and logic for a new FBI headquarters remain undiminished. But making the case for it any time in the foreseeable future will now be that much more difficult owing to Mr. Trump’s toxic commentary and gratuitous disrespect. In an era of heightened domestic security threats, what could be more foolhardy and self-defeating than undercutting the FBI?