NYPD commissioner: "We can never let our guard down"

(CBS News) Several major U.S. cities -- including New York, Los Angeles, and Washington, D.C. -- stepped up security in the hours after the bombing attacks at the Boston marathon Monday. 

New York Police Department Commissioner Ray Kelly said Tuesday on "CBS This Morning," "obviously we're concerned about a similar type event."

However, Kelly said city authorities remain constantly vigilant in their attempt to maintain a "360 [degree] perimeter" around the city at all times and said, "We've had 16 plots against the city since September 11, 2001. In the last six month we've had five individuals arrested or convicted for plotting against New York City. We believe the threat has remained constant since 9/11."

Special section: Boston Marathon bombings
Boston blasts prompt major cities to step up security

While he called NYPD efforts to manage terror threats "very, very challenging," Kelly said his team operates under the premise that "we can never let our guard down."

"We invest certainly more than any city I'm aware of in our counterterrorism efforts but there are no guarantees," Kelly said before adding that the cooperation between federal, state, and local officials in response to attacks like the Boston Marathon bombings functions "as never before."