In two decades since 9/11, lots of things have changed. The constant? Lasting uncertainty.

Critics say the online forum enables the spread of hate speech.

Extremists are far too at home on social media. Whose job is it to root them out?

“Large media corporations have a duty to take on this challenge,” one expert says.

And what can social media companies do to intervene?

"Many people … trust these big tech companies frankly more than they do their politicians," Politico's chief technology correspondent says.

Even years after an event, the trauma lingers.

“You are dealing with people whose lives have been destroyed and money is a pretty poor substitute for loss.”

Travelers, wary of potential terrorism and natural disasters, are increasingly protecting themselves in advance.