Before we get to the dirty bomb stuff, I get to say “I told you so” about where this threat came from. Last night I speculated that it was Younis al-Mauritani, the AQ capo charged by Bin Laden with organizing international plots. He was captured recently in Pakistan — no one’s quite sure precisely when — thanks to a joint CIA/ISI operation. According to an intel official who spoke to Daveed Gartenstein-Ross today, al-Mauritani is indeed the source of the intelligence on the new plot. Former Bush counterterror advisor Fran Townsend also suspected it was al-Mauritani, but she adds a wrinkle:

Source says Pak-US CT relationship still rock bottom. No trust. Al Mauratania arrest was targt of oppty. US gettg no direct access 2 him.

If the U.S./Pakistani relationship is still at rock bottom and we’re not getting these details straight from the horse’s mouth, who knows how much of what Pakistan is telling us — or not telling us — is true. Sending us off on a wild goose chase would be partial payback for humiliating them with the Bin Laden raid. But then, so would holding back key details that might help us break this thing up before it goes off.

Anyway, the dirty bomb. If you’re looking for cheap, relatively easy ways to cripple an already hobbled economy, nothing will do it faster than spreading some cesium all over Wall Street:

The crossroads of the world came to a grinding halt at various times Friday because for police Times Square has become a sort of ground zero in the latest terror threat. Sources told CBS 2’s Marcia Kramer that another attempt to explode a bomb there a key concern, and the intelligence is that it could be a dirty bomb…

Kramer recently talked exclusively with Kelly about the possibility of a dirty bomb, but now sources said the classroom drills might have to be put into practice. Intelligence sources said the terrorists could be trying to make a dirty bomb this time my mixing radiological isotopes with an explosive device.

That’s why the NYPD is deploying all kinds of radiation detectors, panel trucks with sophisticated instruments. On Friday, cops had personal radiation detectors on their belts, and radiation detectors are being used by police within a 50-mile radius of New York City — in New Jersey, Connecticut, and Pennsylvania and on Long Island.

The Daily claims that this one goes right to the top:

The mastermind behind the terror plot announced yesterday was the man who filled Osama bin Laden’s shoes as al Qaeda’s new No. 1: Ayman al-Zawahiri.

In early August, Zawahiri personally recruited three terrorists to travel to the U.S. to perpetrate a string of car-bomb attacks on Washington, D.C., and New York City to coincide with the 10th anniversary of September 11, according to high-ranking sources in the counter-intelligence community who spoke with The Daily on condition of anonymity…

To reach the U.S., two of the operatives traveled to Pakistan through Iran, sources said. In Pakistan they met up with a third operative and traveled to Kabul, Afghanistan. In Kabul they “dropped and swapped” passports and other identification before traveling to Dubai, Saudi Arabia. From there they traveled to the U.S.

I find it hard to believe Zawahiri “personally recruited” anyone. The guy who used to hold his position took a bullet in the face a few months ago; the guy who replaced him as number two was hit by a drone missile within the last few weeks. He has bigger things to worry about right now than staffing, which as I understand it is usually farmed out to the operational guys. Besides, Zawahiri isn’t known for his charm; there are surely more persuasive people than him in the inner circle to make the martyrdom pitch to whichever young jihadi schmuck they’ve got lined up for the next plot. On the other hand, precisely because Zawahiri isn’t Mr. Popularity inside Al Qaeda, he must be looking for some sort of near-term “victory” over America to consolidate his authority within the group. And the fact that the two operatives here allegedly traveled through Iran lends credibility to the idea that this plot is being managed at the highest levels. Remember, before he was killed, Atiyah al-Rahman was AQ’s operational chief and its liaison to Iran, charged with moving jihadis through that country and into Pakistan. Could be that he put this plot together with Zawahiri’s blessing and set it in motion before he died.

Who are the would-be attackers, you say? Details are sketchy, but The Daily hears there are three of them and that two are American citizens, one of Egyptian descent and the other of Yemeni ethnicity. Needless to say, it makes sense that AQ would want citizens for this since it’s easier for them to enter the U.S. More from the Times:

Two senior American law enforcement officials said the tip came from an informant in the Afghanistan-Pakistan region who passed word of the purported plot, intended to coincide with the 10th anniversary of the Sept. 11 attacks, to American intelligence officers on Wednesday. The informant said two American citizens of Arab ancestry had left Afghanistan, traveled through one or other countries and reached the United States as recently as last week.

The officials said they had only a vague physical description of the two men — one described as 5 feet tall, the other 5-foot-8 — and a first name for one of them that is common in the Middle East. The tipster also described a third conspirator, but he appeared to have traveled to Europe…

The other law enforcement official said some of the informant’s account “didn’t seem right.” The official questioned, for instance, why the two men would leave the United States and then come back to carry out an attack, unless they received training in Pakistan. But there is no evidence yet that either man trained there, two officials said.

A WSJ report published last night speculates that there are three Al Qaeda operatives who’d be prime for a mission like this because they’ve lived here before: Adam Gadahn, Adnan al-Shukrijumah, and Jude Kenan Mohammad, who’s suspected of being in Pakistan and under indictment for providing material support to terrorism. Problem is, none of them quite match the descriptions above. Neither Gadahn nor Mohammad are of Yemeni, Egyptian, or other Arab ancestry (Mohammad is Pakistani) and Gadahn’s way too visible for a covert operation inside the U.S. anyway. Shukrijumah is of Saudi descent, but if you’re willing to overlook that discrepancy, he’s a fairly close match to the descriptions. He is of Arab ancestry and he’s also short like one of the alleged attackers (some estimates peg him at 5’3). U.S. counterterror officials have feared him dating back to the days of 9/11 partly because of his interest in — ta da — dirty-bomb plots, and he was linked most recently to the foiled 2009 plot to bomb New York City’s subways on — ta da again — the anniversary of 9/11. He used to live in Florida, speaks excellent English, and can easily pass as Hispanic or Caribbean, so he’s an ideal infiltrator if AQ leadership is willing to part with him. Here’s a selection of FBI photos of him and here’s a video of him showing off his fluency in English. Have a look, just in case.

Exit question: If the dirty-bomb rumors are true, who gave them the radioactive material?