"The question is, when did they become aware of his presence at the airport, and when was he put on the list?" the source told the Post. DHS has not yet confirmed when or whether Shahzad was placed on the no-fly list.
In a press conference moments ago, Attorney General Eric Holder suggested that law enforcement had the situation under control from the start. "I was here all yesterday and through much of last night and was aware of the tracking that was going on," he said. "And I was never in any fear that we were in danger of losing him."
CBS News adds that it was only thanks to a tip from Emirates Airlines that Shahzad, 30, was nabbed. Emirates notified the Department of Homeland Security that it had received a last-minute request for an all-cash purpose of a one-way ticket to Dubai -- all of which are red flags. Before the arrest was made, law enforcement authorities had already notified authorities that the plane was not to take off, CBS added.
From Dubai, it appears that Shahzad intended to continue to Pakistan, from where he had recently returned. Pakistani authorities today arrested several men in connection with the alleged plot.
Had Shahzad made it out of the country -- even only to be arrested and returned here after the plane landed in Dubai -- it likely would have caused severe embarrassment and second-guessing in law enforcement and government circles.
It seems unlikely -- though possible -- that authorities deliberately allowed Shahzad to get on the plane, perhaps in order to gather further intelligence. Typically, law enforcement agencies prefer to arrest a suspect before he gets on a plane, for obvious safety reasons.
Umar Abdulmutallab, who is accused of plotting to detonate a bomb on a Christmas Day flight from Amsterdam to Detroit, was reportedly on a terrorist watch-list maintained by the National Counterterrorism Center, but not on a no-fly list.
Late Update: DHS Secretary Janet Napolitano confirmed today that Shahzad was put on the no-fly list yesterday, ABC News reports. She added that the government only became aware of his presence on the flight -- presumably after being tipped off by the airline -- after the manifest was completed.
ABC offers a potential reason why Shahzad was able to get on the plane: the government only checks people's documents as they enter the U.S., not as they exit. A former Bush administration official told ABC that when counter-terrorism experts recently proposed that we also check people as they're leaving -- as many other countries do -- "the airlines went ballistic," arguing that such step would be expensive and cause delays. So the idea was dropped.