Kenneth Williams

From 9/11 Encyclopedia:

In July 2001, FBI agent Kenneth Williams released the so called "Phoenix-Memo", in which he warned about suspicious activities by Middle Eastern men at an Arizona flight school. But this memo wasn't made public and classified until early May 2002. Then it came out, that another FBI agent, Michael Kennedy did not prioritize this memo and was ignored by FBI-HQ leader Dale Watson (-> ex-CIA), Robert J. Chiaradio and the manager of the office, Dave Frasca (->), who ignored another warning by FBI agent Coleen Rowley (->), too.
Source: http://www.nytimes.com/2002/05/21/national/21INQU.html

Another email, forwarded to FBI-terror specialist John O' Neill (->) didn't help avoiding the attack, too. O' Neill was forced to leave the FBI and started a new job as security chief in the Twin Towers. where he died on September 11th. On May 14th, 2002, it was revealed that the memo cited Osama bin Laden by name and suggested that his followers could use the schools to train for terror operations. This "bombshell" led to the confession of President Bush, that he had prior knowledge on an attack since at least August 6th, 2002 (See Bush Knew). Williams case opened a series of independent articles, which basically proved, that the US government let the September 11th attack happen on purpose (See Lihop). 

On September 20th, 2002, Williams repeated his accusations against former FBI HQ boss Dale Watson. Though hidden by a screen in new 911-hearings, his identity was leaked to the press. In these second 911-hearings, Williams revealed, that he knew thirteen days before September 11th on August 29th, 2002, that "someone Will Die". This info circulated in an e-mail in which he accused the NSLU: "...Thirteen days before the September 11 attacks, a frustrated FBI agent warned headquarters that "someday, someone will die" after he was denied permission to pursue a man who would become one of the hijackers. Let's hope the [FBI's] National Security Law Unit will stand behind their decisions then, especially since the biggest threat to us now, UBL, is getting the most `protection" http://ap.tbo.com/ap/breaking/MGAFZHPQC6D.html 

Williams spoke about Khalid al-Mihdhar (->), that "we had talked about for three months" After Williams accusations on September 21st 2002, only three days later Reuters reported on a bizarre detail: "FBI officials in New York knew that several associates of Osama bin Laden had trained at U.S. flight schools, but believed the al Qaeda leader needed pilots to transport goods in Afghanistan, a congressional investigator said on Tuesday.." http://ap.tbo.com/ap/breaking/MGAFZHPQC6D.html 

Compare: http://www.nydailynews.com/2002-05-20/News_and_Views/Beyond_the_City/a-151521.asp 

'Superstar' FBI Agent Ignored "He is one of the sharpest agents I have ever met," said former FBI agent Ronald Myers, a constable in Maricopa County, Ariz. "He's a superstar. ... Anyone in FBI management who wouldn't take what Ken Williams said seriously is a fool." 

Williams is a former San Diego beat cop who joined the FBI about 10 years ago. "He was a great cop," 
said retired FBI agent Roger Browning, one of Williams' first FBI supervisors. 

Williams served under Browning for about five years on the FBI's SWAT team in Phoenix, and showed the brains as well as the brawn for being a topnotch agent. "He was very understanding and very savvy and almost a mentor to the younger agents on the SWAT team," Browning said.