Both sides must have the same WEP key, which is usually a total of 64 or 128 bits long. A semi-random 24 bit number called an Initialization Vector (IV), is part of the key, so a 64 bit WEP key actually contains only 40 bits of "strong" encryption while a 128 bit key has 104. The IV is placed in encrypted frame's header, and is transmitted in plain text.
Traditionally, cracking WEP keys has been a slow and boring process. An attacker would have to capture hundreds of thousands or millions of packets—a process that could take hours or even days, depending on the volume of traffic passing over the wireless network. After enough packets were captured, a WEP cracking program such as Aircrack would be used to find the WEP key.
Fast-forward to last summer, when the first of the latest generation of WEP cracking tools appeared. This current generation uses a combination of statistical techniques focused on unique IVs captured and brute-force dictionary attacks to break 128 bit WEP keys in minutes instead of hours. As Special Agent Bickers noted, "It doesn't matter if you use 128 bit WEP keys, you are vulnerable!"
On With The Show
Before we get into the steps that the FBI used to break WEP, it should be noted there are numerous ways of hacking into a wireless network. The FBI team used publicly available tools and emphasized that they are demonstrating an attack that many other people are capable of performing. On the other hand, breaking the WEP key may not necessarily give an attacker complete access to a wireless network. There could also be other protection mechanisms such as VPNs or proxy servers to deal with.