Many people had Prior Knowledge of 9/11, and some tried to profit from it.
Stock market traders knew in advance:
Unknown individuals -- with accurate foreknowledge of the attacks -- had purchased an obvious and unusually large number of "put" options on United and American Airlines shortly before the attacks. See InsiderTrading.
The FBI knew in advance:
The day before 9/11 the FBI closed down 500 Muslim web sites:
Five hundred websites - many of them with an Arab or Muslim connection - crashed last Wednesday when an anti-terrorism taskforce raided Infocom Corporation in Texas. The 80-strong taskforce that descended upon the IT company included FBI agents, Secret Service agents, Diplomatic Security agents, tax inspectors, immigration officials, customs officials, department of commerce officials and computer experts. Three days later, they were still busy inside the building, reportedly copying every hard disc they could find. Infocom hosts websites for numerous clients in the Middle East, including al-Jazeera (the satellite TV station), al-Sharq (a daily newspaper in Qatar), and Birzeit (the Palestinian university on the West Bank). It also hosts sites for several Muslim organisations in the United States, among them the Islamic Society of North America, the Muslim Students Association, the Islamic Association for Palestine, and the Holy Land Foundation for Relief and Development. [ www.guardian.co.uk/elsewhere/journalist/story/0,7792,549590,00.html The Guardian, Monday, September 10, 2001.]
Some of the Twin Towers tenants seem to have known in advance:
Government Had Prior Knowledge - Pending 9-11 Attack Oklahoma City
Bush Knew - 9/11Encyclopedia
Hauer, Jerome - 9/11 Encyclopedia
Flashback: FBI Probes Espionage at Clinton White House - Israeli spy