U.S. Immigration and Customs Enforcement officers arrest suspects. (Charles Reed/U.S. Immigration and Customs Enforcement via AP)

THE TOP official at Immigration and Customs Enforcement, whose stock in trade is tough-guy double-speak on deportation, occasionally wanders into the territory of outlandish falsehoods, often in the service of the idea that Americans should be very afraid. That was the case last month when Thomas D. Homan, ICE’s deputy director (and, in the absence of a director, the agency’s de facto No. 1) suggested that more than 800 “criminal aliens” were at large in Northern California because the mayor of Oakland had tipped them off.

That was a risible exaggeration apparently triggered by Mr. Homan’s wrath that the mayor, Libby Schaaf, a Democrat, had announced that ICE planned raids to arrest undocumented immigrants across the Bay Area in late February. It was also the sort of blithe fiction the Trump administration usually peddles without public pushback from career civil servants. Not this time.

This week, ICE’s own spokesman in San Francisco resigned in protest of Mr. Homan’s false assertion, which was seconded by Attorney General Jeff Sessions. “I quit because I didn’t want to perpetuate misleading facts,” James Schwab, the ICE official, told the San Francisco Chronicle. He said that when he told his superiors that “the information was wrong, they asked me to deflect, and I didn’t agree with that.”

Ms. Schaaf issued her warning that ICE raids were imminent on Feb. 24, just before agents fanned out in search of some 1,000 illegal immigrants with misdemeanor or felony records. Her action was ill-advised; while the mayor said she objected in principle to sweeps that would inevitably ensnare some immigrants with unblemished records, she might have put some agents in needless peril.

In any event, however, the ICE raids were more or less successful, producing 232 arrests of undocumented immigrants, half of them with criminal records. That’s roughly in line with expectations; deportation sweeps generally net just a fraction of the targeted suspects.

Despite that, Mr. Homan blamed the mayor’s warning for allowing some — or even all, as he said a day later — of 800-odd immigrants to escape capture. Mr. Sessions then fanned the fires, asserting, “Those are 800 wanted criminals that are now at large in that community . . . all because of one irresponsible action.” That was plain nonsense, as Mr. Schwab pointed out.

The broader context for the incident is the administration’s fury at so-called sanctuary cities such as San Francisco and Oakland, where officials limit cooperation with ICE deportation agents in the interest of maintaining ties with their immigrant communities. In some cases that refusal to cooperate is ill-advised, but often even sanctuary jurisdictions hand over dangerous felons who have served their sentences. Those who are shielded, in the sense that they are set free after serving jail time, tend to have committed less-serious crimes.

Heedless of such distinctions, immigration officials such as Mr. Homan appear to believe they are involved in a sort of war. And in war, the first casualty is invariably the truth.