LeT operative David Headley's cellphone and his Chicago apartment were registered in the names of dead persons and investigations have found that he was leading a double life under directions from the Pakistan-based terror outfit, according to the FBI.
Headley changed his name from Daood Gilani in 2006 to further his cover, the FBI said.
A team of officials from FBI and the Justice Department headed for Pakistan from Delhi to follow up on leads after briefing Indian authorities on its investigations into the conspiracy involving Headley, a Pakistani-American, and his Pakistani-Canadian accomplice Tahawwur Rana, in the Mumbai terror attacks.
"We have been having close consultations with Pakistani authorities in this investigation and the joint FBI-Department of Justice team will meet officials of various government agencies," US Embassy spokesman in Islamabad Rick Snelsire told PTI.
In New Delhi, Home Secretary GK Pillai said the terrorists were working on new strategies and the time has come to "think the unthinkable".
"As we have seen from the Headley case that one can still slip through the radar...Because nobody will look for a US citizen with an American passport and a business visa".