Up to a dozen others may be involved in the alleged bomb plot, officials say. Police nationwide were told Tuesday to look out for suspicious activity at subway stations and stadiums.
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.