The U.S. has done a lot to make itself safer since 9/11, but a new report shows the New York City subway system is one of the country's most vulnerable targets.
The report points to the system's underwater tunnels as a major concern, saying if they were attacked, water would fill-up the subway system killing an estimated 19,000 people.
Other cities like Washington, D.C. and Atlanta have advanced surveillance systems in place to stop attacks.
New York City has been working on such a system for the last four years, but it is being held up by legal issues.
"I think the Secretary of Homeland Security should have on her desk a list of the high priority projects that are in trouble and this would be up if not on the top of that list," counter-terrorism expert Richard Clarke said.
Homeland Security Secretary Janet Napolitano told reporters the "overall safety of the subways is safe," but she did not comment on technology advancement.
The report said an attack on New York's subway would not only kill tens of thousands of people, but also shut down the subway system for years.