One day: Sept. 11, 2001

>>> every so often, we're forced to remember in detail what happened on 9/11. tonight, it's probably useful to remember what osama bin laden did to us here. nbc's rehema ellis at ground level with more on that. good evening.

>> reporter: good eve rng brian. on 9/11, lives changed dramatically. today, the news about osama bin laden stirred up painful memories that for many people never go away. the initial reports on that crisp fall morning were at first unclear.

>> we have a breaking news story to tell you about. apparently, a plane has just crashed into the world trade center here in new york city .

>> no one could have imagined what would unfold.

>> after the first crash into the north tower at 6:00 a.m ., word of a second plane into the south tower at 9:00 a.m ., and the horrifying picture seemed to emerge.

>> you have to move from talking about a possible accident to something that has happened there.

>> reporter: and then a third plane. this crashed into the pentagon.

>> people are describing this as a highly sophisticated, coordinated attack.

>> reporter: america was under attack.

>> it goes without saying this is the most serious attack on the united states in more than 100 years.

>> reporter: in lower manhattan , chaos and terror.

>> crazy, the craze ystz thing i have ever seen in my life.

>> reporter: we would soon learn a plain that crashed near shanksville, pennsylvania, was headed for the capitol, but it was overtaken by passengers and forced down.

>> the best thing to do would be to stay home.

>> reporter: president bush spo spoke to the nation.

>> terrorism against our nogz won't stand.

>> reporter: they were vulnerable. were you scared for your life?

>> hysterical.

>> reporter: to the first responders, the definition of brave changed forever on september 11th , 2011 . there is still pain --

>> no one should have to die like that.

>> reporter: but also, a sense of a new beginning.

>> now that he's gone, the sun has come out and the cloud is gone.

>> reporter: and time seemed to stand still on september 11th , but now, each new day seems to bring just a little more healing. brian.