ONE FBI MEMO, written by a Phoenix agent in July 2001, warned about suspicious activities by Middle Eastern men at an Arizona flight school. Last week, in little-noticed testimony before a Senate panel, FBI Director Robert Mueller referred to another internal document that may prove more explosive: notes by a Minneapolis agent worrying that French Moroccan flight student Zacarias Moussaoui might be planning to “fly something into the World Trade Center.” The notes are especially eerie because Moussaoui faces charges that he was part of the 9-11 plot. Sources say the notes Mueller referred to were written in early September 2001—days before the attack. The author was part of a counterterrorism team desperately trying to figure out what Moussaoui was up to. He had been arrested in August on immigration charges after a Minnesota flight instructor reported that he showed a suspicious interest in learning how to steer large airliners. When agents learned, from French intelligence, that he had radical Islamic ties, they sought a national-security warrant to search his computer—and got turned down. From his e-mail traffic they found he wanted to learn to fly a 747 from London’s Heathrow to New York’s JFK. The agents held “brainstorming” sessions to try to figure out what targets might be en route. The agents were “in a frenzy,” “absolutely convinced he was planning to do something with a plane,” said a senior official. One agent wrote that “one possibility” was that Moussaoui might be planning to crash into the Twin Towers. But the official said the agents were only “speculating” about possible scenarios. Congressional investigators believe there are more embarrassing documents to come. Another sensitive issue: the CIA’s failure to aggressively follow up on information provided by Malaysian authorities in January 2000 about a meeting in Kuala Lumpur of Al Qaeda operatives—including two men who turned out to be among the 9-11 hijackers. Malaysian officials passed along photos to the United States, but they never heard back and stopped monitoring the suspects, one Malaysian official told NEWSWEEK. CIA officials said the significance of the meeting didn’t become clear until much later. But by the time the CIA alerted the FBI, it was too late.