Boston Metro Area on Lockdown; Transit and Security Precautions Not Seen Since 9/11 Attacks

The city of Boston and surrounding neighborhoods are essentially on lockdown at this hour. State and local officials have asked people to remain in their homes with their doors locked and only to answer to a properly identified law enforcement officer, as police fan out across the city hunting for one of the two Boston Marathon bombers. Transit in the Boston area has been halted. The FAA has cleared the airspace over the city. Taxi cab service also has been stopped. 

Not since the 9/11 attacks have widespread transportation stops been put into effect like this in a major US city. Television reports show SWAT teams, heavily armed police forces, and counterassault vehicles on the streets of the Boston area. Authorities have moved in large numbers towards two addresses. One is the apartment at 410 Norfolk St., in Cambridge, where the two bombers are believed to have lived. 
Another location appears to be at the corner of School St. and Cypress St. in Watertown. CNN showed images of law enforcement officers, weapons drawn and wearing body armor, standing on the roof of a local restaurant and peering into the windows of a nearby building. A CNN reporter on the scene said she smelled smoke, and soon after police moved in. A police helicopter is reportedly hovering over the scene. 
Following the 9/11 attacks, transit in the Washington area also stopped. Police and national guard forces moved out across the city as officials feared follow-up attacks could be imminent.