It happened overnight, according to Brian Ross, Richard Esposito, and Clayton Sandell of ABC News.

The leader of a New York mosque, and two Colorado men were arrested by the FBI. It appears from the evidence that has been released that the father and son - Mohammed and Najibullah Zazi of Denver - as well as Ahmed Afzali in New York were all involved in an unspecified terrorist plot.

In New York, the FBI arrested the leader of a Queens mosque, Ahmad Afzali, who authorities allege had been a New York police department informant but "went bad" and tipped off Zazi, his father and others to the investigation.

Afzali's alleged double cross compromised the case and led the FBI to move in on the suspects much earlier than they had wanted and led to extreme tensions between the Denver office of the FBI and the NYPD. One law enforcement official said the NYPD had been specifically asked not to reveal the investigation to its informants, but went ahead anyway.

n affidavits filed in connection with the arrests, agents say they discovered nine pages of handwritten notes in Zazi's computer with details on how to make a homemade bomb.

According to the affidavit, Zazi initially denied knowing anything about the notes when he was shown them by the FBI during a three day interrogation.

[...]

Law enforcement officials told ABCNews.com that electronic intercepts revealed Zazi had sent text messages suggesting, in code, the plot was nearing the attack stage. "The wedding cake is ready," Zazi allegedly wrote. 

The fact that Afzali tipped the Colorado men probably means we will never find the bomb they were going to use. It also means that the real "Catch-22" regarding when to move in and make arrests in these cases will further muddy the waters, as Rusty Shackleford writing in PJ Media today explains:

If you intervene too early, a group of serious terrorists intent on doing real harm may appear to be only a bunch of kids with fantasies full of grandeur and a lot of bravado. The weak evidence of an actual criminal conspiracy in these cases may lead prosecutors to seek lesser charges such as "lying to immigration officials."

By nipping a terrorist plot in the bud we are saved from the consequences of acting too late (another 9/11). In doing so we also run the risk of feeding a preexisting fear in the Muslim community that they are being unfairly targeted by law enforcement. This, in turn, may lead to less cooperation from the very community we need the most in preventing a terror attack. It also means that the next 9/11 might go undetected because members of the Muslim community don't take the threat seriously.

This is the post-9/11 Catch-22.