From the story:
All four are understood to be Asians living in London and are feared to have links both with Islamic extremists in Britain and worldwide terror groups -- including al-Qaeda training camps in Pakistan and Afghanistan.
MI5 chiefs reportedly believe the suspected moles have been planted as sleepers -- agents under deep cover -- to keep al-Qaeda informed of antiterror raids planned by London's Metropolitan Police.
They are said to fear the four could have already accessed sensitive information about secret operations to root out terror cells planning further attacks in the U.K.
This goes to show that as much as governments, and even businesses, focus on outsiders, better precautions need to be put into place to ferret out insiders gone bad in the first place. Some of the worst examples of insiders inflicting enormous harm on the United States have been our own agents, such as Robert Philip Hanssen at the FBI and Aldrich Ames at the CIA.
The challenge is, how do you find these threats before they do their harm?