Getting agents into Islamic terrorist organizations has proved very difficult. There are several reasons for this. First of all, most terrorists come from cultures where the language spoken (Arabic, for example) is difficult to learn, and there are not many speakers of these languages that could otherwise qualify for a job in the CIA or FBI. Then there are the problems with background checks. Islamic terrorists are big on background checks. Not only do they know how to use Google, but they will also, eventually, send someone to your "hometown" to ask questions. The CIA has discovered that, Google alone has made it much more difficult to create "legends" (the fake background information agents require). But the discovery that al Qaeda will even send people out to check legends, was really scary. With all that, al Qaeda is also very patient, and is willing to wait years, before giving a new guy more access to top people, and more important information.
As a result of all these obstacles, the CIA has found it more effective to put more effort into recruiting existing terrorist leaders. Turns out that corruption has long been a problem with al Qaeda. While al Qaeda exists largely to fight the corruption of Moslem leaders, they have not been able to completely rid themselves of larcenous and deceptive habits. Among the many al Qaeda documents captured in Afghanistan, there were quite a few letters and reports dealing with embezzlement and other evidence of corrupt leadership. Apparently, many al Qaeda regulars eventually get tired of the life. It's a hard life, and often a very precarious one. Despite the culture of suicide and sacrifice within al Qaeda, many of the members have families, and attachments develop. Some members change their minds, and this provides opportunities for any CIA people who have some channels open to these guys. Opening up these channels, which allows some messages to be sent either way, is now a major effort. This sort of thing can be moved along quickly via the use of lots of money. You can get to the non-terrorist kin of al Qaeda members, or tribal leaders, by using cash. You can see where this is going. If you can't easily train agents to spy from inside al Qaeda, then work on hiring someone who is already there. Apparently, several of the al Qaeda big shots who were taken down, went partly because of information from an al Qaeda member that was already bought, or on short-term lease.