Anwar al-Awlaki, who was killed last year in a U.S. drone attack in Yemen, had as many as 60 e-mail addresses, retired FBI agent Keith Slotter told Fox. Slotter said that when Awlaki realized he was on the kill or capture list, he avoided telephones and rotated his e-mail addresses.
"He'd let some go dark, and he'd use 10 or 15, and then those would go dark, and he'd go to a different set. So he was constantly revolving," Slotter, who now works for a security firm, said. "As you can imagine with that many accounts, it was quite a lot to stay on top of."
Awlaki sent 19 e-mail messages to Maj. Nidal Hasan, the U.S. Army psychiatrist charged with gunning down 13 people at Fort Hood.
"There was nothing really in there that would indicate al-Awlaki prompting Major Hasan to do something," Slotter said.
British documents, which Fox said it obtained through the Freedom of Information Act, said Awlaki gave detailed instructions, in encrypted messages, to Rajib Karim. Karim was sentenced to 30 years in Britain for plotting a terrorist attack.
The first episode of "Fox Files: The Enemy Within" is to air Friday night on the Fox News Channel.
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