Terror plot: a failed replay of London and Madrid?

More arrests are likely in the alleged Al Qaeda plot on American targets that appears to be the first operational plan to execute mass-casualty attacks within the US since 9/11.

Law enforcement officials have told reporters that up to a dozen others may be involved in a plan to bomb subway stations and other crowded venues. On Tuesday, the Department of Homeland Security and Federal Bureau of Investigations issued bulletins to local police to watch for suspicious activity around stadiums, entertainment complexes, and hotels, according to the Associated Press. The agencies said they had no information on timing or location but that it was "prudent to raise the security awareness of our local law enforcement partners regarding the targets and tactics of previous terrorist activity."

Previously, a national alert was issued about people purchasing large quantities of chemicals that could be used for explosives.

While evidence is still emerging in the case around Denver airport shuttle driver, Najibullah Zazi, experts and officials say the details suggest the plan is similar to attacks in London and Madrid in which suicide bombers killed scores of people.

Federal agents "found things that ... suggest there may have been methods here that are similar to previous attacks," says Daveed Gartenstein-Ross, a terrorism expert and vice president of research at the Foundation for Defense of Democracies in Washington. "There is a belief that these guys were serious."

In the 2004 Madrid bombings, which killed 191 people, the attackers blew up four commuter trains. In London, 52 people died after four men detonated peroxide-based bombs on buses and trains during rush hour in July 2005.

In this case, law enforcement officials suspect that Mr. Zazi and others also intended to build peroxide-based bombs. Federal agents have apparently discovered bomb-making instructions on Zazi's laptop and he has reportedly admitted to receiving weapons and explosives training at an Al Qaeda camp in Pakistan.

Zazi is currently being held on charges that he lied to federal authorities, and is likely to be charged with supporting a terrorist organization, based on his admission of training in Pakistan. His father, Mohammed Wali Zazi, and a New York imam, Ahmad Wais Afzali, are also being held in connection with the case.

Zazi has denied being involved in any terrorist plot.

Mr. Gartenstein-Ross says he isn't necessarily surprised about the recent revelations. "There is nothing surprising about this. It's more surprising that you haven't seen more like this," he says. "We've been blessed compared with Europe."

Earlier this month in London, three British-born Muslims were convicted of plotting to explode North America-bound airplanes in 2006. The scale of that plan rivaled 9/11 and the convictions were the culmination of Britain's largest terrorism probe.

That case was also disturbing because it did not involve immigrants but Muslims born in Britain. While Zazi is originally from Afghanistan and lived in Pakistan for a time, he moved to the US a decade ago and is a legal resident. That fact troubles counterterrorism officials, according to The New York Times. "Zazi embodies what concerns them most: a Westernized militant, trained by Al Qaeda in Pakistan, whose experience and legal resident status in the United States give him the freedom to operate freely," the paper wrote.

According to the Denver Post, Zazi returns to court on Thursday for a preliminary hearing.

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