Federal officials issued a warning Tuesday to mass transit stations across the country after three suspects were arrested in a possible terrorism plot that may have targeted New York City trains.
The plot may have been similar to the attacks on London and Madrid's subway systems, in which bombers would have used backpack bombs.
Officials have recommended random bomb sweeps at terminals and stations.
Click play for further analysis of the terror warning from CBN News Terrorism Analyst Erick Stakelbeck.
Meanwhile, two out of three terrorism suspects are being held without bail in Colorado and New York, in what has been called a varsity level terrorist plot.
This includes a man born in Afghanistan, who has admitted to having connections to al Qaeda.
New details emerged on Monday, surrounding the arrest of three men said to be connected to a thwarted terrorist plot targeting New York City.
FBI officials charged 24-year-old Najibullah Zazi, his father Mohammed Wali Zazi, and New York Islamic leader Ahmad Wais Afzali with lying to federal agents. The men appeared in court Monday.
Najibullah Zazi denied any ties to al Qaeda, but later came clean with federal authorities. He now admits receiving explosives training from al Qaeda in Pakistan.
Investigators believe Zazi intended to put that training to use in what they call one of the most serious plots against U.S. targets since 9/11. So far, however, no additional charges have been made against him.
Authorities found bomb-making documents on Zazi's computer, along with nine pages of hand written notes. When he was questioned about and shown the notes, however, he denied any connection to them.
Zazi and his father were arrested last week in Denver, Colo., and charged with making a false statements to federal agents. Afzali, the leader of a Queens, N.Y., mosque faces the same charges.
Afzali tipped Zazi off that he was being watched, then lied about it to agents. Ironically, Afzali worked as an informant for the bureau.
Authorities say they do not know the specific targets of the alleged plot, but information about several New York City landmarks was found on Zazi's computer. Grand Central Station and football and baseball stadiums were among the sites.
"America is constantly under threat," said New York City Mayor Michael Bloomberg. "We are a target, and particularly New York City."
The decision to move in on Zazi came after he sent a text message to associates in the U.S. that read "The wedding cake is ready."
Authorities believe that was code for an impending attack.