Two years ago, it was revealed that the U.S. FBI (Federal Bureau of Investigation) was using a hacker tool to legally (with a court warrant) monitor what was happening on the target computer (of a suspect). This came as little surprise to criminals, and especially terrorists. It's long been known that law enforcement agencies have used computers to catch criminals and terrorists. This was largely thought to be done with taps into Internet traffic through various chokepoints (local, via ISPs, and nationally via major fiber optic cable systems). But it was also suspected that that the intelligence agencies and police were using hacker tools. Many terrorists have reported, on pro-terrorist message boards, finding secret hacker software planted on their computers. But no one had much definitive proof that it was from, say, the CIA, MI6, Mossad or the FBI. Most of it appeared to be the usual criminal stuff.
For nearly a decade, cyberwar and criminal hackers have planted programs ("malware") in computer networks belonging to corporations or government agencies. These programs, called "Trojan horses" or "zombies", are under the control of the people who plant them, and can later be used to steal, modify or destroy, data or shut down the computer systems the zombies are on. You get the zombies secretly placed using freshly discovered, and exploitable, defects in software that runs on the Internet. These flaws enable a hacker to get into other peoples networks. Called "Zero Day Exploits" (ZDEs), in the right hands, these flaws can enable criminals to pull off a large online heist, or simply maintain secret control over someone's computer.
Now, it's generally accepted by terrorists that the intel agencies are seeking to infect their PCs with zombie type software. Most terrorist computer users are low tech guys, and there are not enough tech gurus in the ranks to insure that everyone's PCs are kept "clean." While the more serious terrorists insist that everyone keep "business" off cell phones and computers, especially those hooked up to the Internet, not everyone is that dedicated. So the intelligence agencies still obtain lots of useful information via hacker attacks.