TODAY | April 17, 2013
>>> in the wake of the tragedy we're seeing added security, extra vigilance in cities across the country. nbc's tom costello is near the white house with that aspect of the story.
>> reporter: good morning. as you know pennsylvania avenue right in front of the white house is normally closed to traffic, only open to pedestrians but this morning pennsylvania avenue is closed to pedestrian as the fbi hunts for whoever planted those bombs. at the white house , the flag at half staff this morning with security tightened all across washington while a full continent away, flags also at half staff at the lapd nerve center where they focus on intelligence and network of city cameras, big events like the oscars or dodger game are a constant security concern.
>> the public has to help. if you see something, you have to say something. where large crowds are gathered when you see something that's suspicious, please report it to law enforcement . this is all of our responsibility.
>> reporter: worldwide cities are today reexamining their security plans after the boston attack. in london this morning it's the funeral for prime minister thatcher. this weekend, it's the london marathon .
>> it's a very reasonable response to an event such as we've seen. over 170 people injured, three people dead. we'd be professionally irresponsible if we didn't take some reasonable steps.
>> reporter: at wrigley field in chicago last night, increased security for the cubs game, garbage cans that could hide explosives removed. in washington defiance as the capitals took on the maple leafs .
>> you can't live your life hiding under the covers.
>> i'm not going to let fear ruin my life.
>> there's no reason why you shouldn't go out and feel safe and enjoy the freedoms that have been fought for for 230 years.
>> reporter: but there is a security rethink under way for the oklahoma city marathon two weeks away. there'ses about a bike race in rueston, a 10k in miami, nascar in virginia, the kentucky derby and the indy 500 . from police departments across the country, new york to chicago, atlanta to l.a., a show of force and the nation's airports, more passenger scrutiny from the tsa, all of it a reality check .
>> i think this is the reality we've been living with for the past 12 or 13 years but it's very easy for people to forget how close we've been before.
>> reporter: police want to reiterate this point, if you have any, any tiny clue, not necessarily even about this case but any case, call it in. they point out that the tiniest clues very often can break a case and can lead to the arrest of a suspect. savannah, back to you.
>> tom costello in washington thank you.