Monday, Aug 2nd:
"The information that led authorities to issue an unprecedented warning of potential terrorist attacks on financial centers in New York, Washington and New Jersey came from at least three al-Qaida members who were deemed highly reliable, U.S. and Pakistani officials said Monday.
Two were pivotal al-Qaida figures arrested last month in Pakistan in separate raids where documentary evidence also was captured, the officials said. The more important of the two was an al-Qaida computer engineer who relayed communications to the network's members, they said.
The third is a member of Osama bin Laden's terrorist network who's in British custody and is cooperating, one official said."
http://www.sanluisobispo.com/mld/sanluisobispo/news/local/9304358.htmAlso Monday, Aug 2nd (as dated by Google News; article says Tuesday 3rd August, so probably this was in the early editions of the Tuesday newspaper - still before the arrest of 13 people (at least 4 since released) in Britain:
"The American move follows the arrest in Pakistan of Tanzanian-born Ahmed Khalfan Ghailani, who is wanted in connection with al-Qaida attacks on US embassies in East Africa in 1998, and the capture of a Pakistani computer expert, Muhammad Naeem Noor Khan, a few days earlier.
...
A former senior CIA official said corroborating evidence of a planned attack in the US came from British intelligence, based on the interrogation of a third al-Qaida suspect in its custody. The identity of the suspect could not be confirmed."
http://www.guardian.co.uk/usa/story/0,12271,1274929,00.htmlSo, first to be revealed was Ghailani, on Thursday 29th July, apparently to push the Kerry speech out of the headlines (see the earlier New Republic article) - he was arrested on the previous Sunday.
Then the arrest of Ghailani was said to have uncovered details leading to the New York alert (which might make al Qaeda members worried he'd talked and/or his computer had been deciphered), and Khan's name was given out on Monday 2nd July. We now know that Khan had cooperated by sending emails to people in Britain after his arrest, so that really would alert them that they were close to being arrested.
Also, we have 2 newspapers talking about an al Qaeda suspect in Britain cooperating in interrogation. I don't know whether this person has been under arrest for long; but the news that they're cooperating is also going to alert anyone they work with to get out quick.