Matt Bevan

As New Yorkers came to terms with a possible act of terrorism on US soil, investigators released the first pictures of the explosion site, leaving little doubt that a powerful bomb was the most likely cause.

A van packed with 550 kilos of explosives was driven into an underground car park of the World Trade Centre.

It blasted a hole through three floors, killing six people instantly and injuring hundreds more.

For some time, it wasn't clear who was behind the attack.

As many as 19 groups have claimed responsibility for the explosion, but the mystery of who really did it remains unsolved.

One of the FBI agents involved in the investigation was John O'Neill.

He'd always wanted to be in the FBI, ever since he watched this imaginatively titled TV show back in the 1960s.

The people behind the 1993 attack were religious extremists from Pakistan, and John was instrumental in their capture.

He became obsessed with researching Islamic extremism, and quickly became an expert on the growing threat of terrorism on American soil.

As he continued rising through the ranks of the FBI's counterterrorism department, he was cognizant of the importance of balancing the need to keep people safe with the infringement on their personal privacy.

Ordered liberty.

A balance between government surveillance and individual freedom.

In the US, the government surveillance side of the seesaw is populated by lots of different intelligence agencies.

There's local police and the FBI keeping an eye on people domestically, and the CIA keeping tabs on potential threats abroad.

But in the 1990s, that system had a serious flaw.

Each of those agencies gathered their own data, but sharing data between them was complicated.

To understand how bad the data sharing was, think about information being like marbles in jars.

The FBI has a jar of marbles.

The CIA has one.

And the local police have one.