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.

CNN: Officials: Bin Laden message to al-Zarqawi intercepted

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