The online posting on Aug. 30 sounded like the rantings of a crank: The subject was ''911,'' and it warned ''Something is going to happen tomorrow . . . REPENT!''

On Sept. 4, the author of the first message, ''Xinoehpoel,'' was back: ''Wait 7 days,'' he wrote.

The few people reading the obscure Internet discussion over the prophecies of Nostradamus dismissed it. But seven days after the message, on Sept. 11, the World Trade Center and the Pentagon were attacked. Xinoehpoel quickly returned to the discussion to gloat that he had predicted the disaster.

That was when the F.B.I. and antiterrorism investigators in 10 cities started calling the offices of O1.com, a Sacramento company that sells Internet access to smaller Internet service providers. Xinoehpoel's messages could be traced back to one of the company's clients, said Brad Jenkins, the company's president.

When the subpoenas came, Mr. Jenkins said that he acted personally to make the process of handing over information go quickly and smoothly: ''With this one, we said, 'Don't send 'em through the hoops.' ''