The terror alert system, however, was not in effect at the time, but rather, implemented in 2002.

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With Europe under an unprecedented terror alert, the U.S. State Department has issued a travel alert, warning of potential terrorist attacks.

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As we've been reporting, in the wake of today's bombings in London, US officials have raised the terror alert level for mass transit systems.

NPR: New York, Los Angeles Step Up Security

Outgoing Homeland Security Undersecretary Asa Hutchinson said federal authorities were not ready "to shake up America on this" by raising the nation's terror alert level above its current status of yellow, or elevated.

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Despite the heightened terror alert, the Transportation Security Agency expects the initiative, dubbed the "Registered Traveler" program, to roll out to between 10 and 20 U.S. airports by the end of the year, according to the agency's spokeswoman Amy Kudwa.

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New York has been on "orange" alert, or the second-highest level -- indicating a high risk of terror attack -- since the color-coded warning system was established after the September 11, 2001 attacks.

CNN: New York subway system on alert