Connecticut Senator Joe Lieberman, chairman of the Senate Homeland Security Committee, said yesterday that America's trains and buses are the most "vulnerable" forms of transportation for the kinds of terror attacks seen around the world, most recently in Moscow. Though the Department of Homeland security is introducing enhanced aviation security measures such as the "use of explosives trace detection, advanced imaging technology, canine teams, or pat downs," Lieberman says that more needs to be done to protect public transit. "The threat is real to non-aviation transportation. All you've got to do is look around the world," he told David Gregory on "Meet the Press."

Lieberman said that the government is working with both state and local officials to improve subway safety, but that it's not enough yet. "We, frankly, need to give it more than we're giving it now to protect the American people. I worry about this." New York has already been stepping up subway security in the wake of the Moscow attacks, deploying both uniformed and undercover officers to subway stations around the city.

Mayor Bloomberg said in a press release, "We will learn from the terrible tragedy in Moscow, as we do from every terrorist incident around the world, and we will continue to do everything possible to protect our transit system—and our entire City—from the threat of terrorism." Last week's Moscow metro bombings killed at least 40 people, and subway bombings in London, Madrid, and Mumbai have all happened within the past six years.