Terror plot investigation continues to mushroom

In connection with the Zazi case, federal investigators are combing records on the purchases of chemicals, particularly hydrogen peroxide, that could be used to make bombs. Peroxide-based explosives were used in a deadly attack on London's subway and bus system in July 2005, and al Qaeda's interest in such explosives has been a top concern for the FBI, according to intelligence assessments made public.

Two U.S. officials said key allegations in the case are more reminiscent of the London and other European plots than any other case seen to date in the U.S. "This appears to be different than any of the previous cases because it looks very similar to what we've seen recently in Europe," one of the officials said.

Current and former officials say Mr. Zazi's profile is also a factor leading them toward their conclusion of an al Qaeda link.

"The best chance for success [al Qaeda leaders] have is to get somebody who is not a visitor, so they come in as a permanent resident or citizen, who knows the culture and how to move around, who doesn't have a record, so as not to get picked up in a database," said a former senior U.S. official. Mr. Zazi is a legal permanent resident of the U.S.