From the Detroit News:
The State Department didn't revoke the visa of foiled terrorism suspect Umar
Farouk Abdulmutallab because federal counterterrorism officials had begged off
revocation, a top State Department official revealed Wednesday.
Patrick F. Kennedy, an undersecretary for management at the State Department,
said Abdulmutallab's visa wasn't taken away because intelligence officials
asked his agency not to deny a visa to the suspected terrorist over concerns
that a denial would've foiled a larger investigation into al-Qaida threats
against the United States.
"Revocation action would've disclosed what they were doing," Kennedy said in
testimony before the House Committee on Homeland Security. Allowing
Adbulmutallab to keep the visa increased chances federal investigators would
be able to get closer to apprehending the terror network he is accused of
working with, "rather than simply knocking out one soldier in that effort."
This is really all that we need to know. But the evidence of the Haskells
about how he was helped to get onto the flight by a smartly dressed man (which
has not been widely reported) is also of interest:
http://www.mlive.com/news/detroit/index.ssf/2009/12/flight_253_passenger_says_at_l.html.