The response was broad and deep—and almost instantaneous—because President Obama decided, with the 9/11 anniversary looming on Sunday, that there wasn’t time for slower and more deliberate sifting of evidence. A “For Official Use Only” bulletin was sent to law enforcement around the country Thursday afternoon, and the details leaked almost immediately, with Biden warning citizens to be “vigilant” for something suspicious.

There were no information “stovepipes,” as in 2001. This time, the entire country is being asked, in effect, to “connect the dots.” This flat, real-time dissemination of information may make people nervous, but Obama was right to share information quickly—so people could be on the lookout. That’s what helped get Britain through the wave of IRA bombings in the 1970s and ‘80s, when every subway ride was an adventure, and it makes sense now.

The Wednesday night threat report broke what had been an eerily quiet environment, as intelligence agencies looked for a threat but didn’t find anything.

The dragnet began after the May 2 raid on Osama bin Laden’s compound. In the library of digital information taken from Abbotabad was specific discussion between Bin Laden and his chief aide, Atiya Abd al-Rahman, about a big attack on the U.S. to coincide with the tenth anniversary of Sept. 11.

Soon after, White House counterterrorism chief John Brennan began holding meetings every week or two on threats to the homeland. He and the intelligence analysts sifted potential threats to transportation, infrastructure and other targets. As part of this review, intelligence agencies pulsed their “liaison” counterparts around the world, asking if they had noticed anything unusual or surprising. The FBI did the same thing with its network of informants inside the U.S.

That global investigation didn’t produce any serious leads, and the review process culminated in a meeting Tuesday at the White House. At that point, there was still no “credible threat,” so officials focused instead on the possibility of a “lone wolf” attack by an aggrieved Muslim extremist.

The uncoordinated solo bomber or shooter may still be the most likely danger. But Wednesday night’s intelligence report gave everyone something more specific to worry about. Intelligence officials are understandably guarded in talking about their information, but it appears that the tip came from a source in Afghanistan who has knowledge about Al Qaeda, and in the past has provided reliable intelligence. The CIA’s source said that three operatives had entered the United States in September, with the intention of detonating a car or truck bomb in New York and Washington to mark the 9/11 anniversary. No specific targets were cited, beyond the two cities.

Obama asked the intelligence community Thursday to go back and double-check all its best counter-terrorism sources again, in light of the credible threat. The FBI is doing the same. Biden’s advice Friday morning to the public, on the “Today” show: “Be vigilant. Be vigilant. But continue life as normal. But be vigilant. Report. Report anything that looks suspicious to you.”

“Heard, understood, acknowledged,” as they say in the military. If you lived in Baghdad or Lahore—or London of 20 years ago—this sort of threat vigilance would seem almost routine. It’s still a shock to the American system, and in that sense, it’s a powerful evocation of the anxiety and solidarity of 9/11 itself. This time, we are all on the team that’s looking for the terrorists.