The following link directs you to portions of the US Department of Homeland Security`s (DHS) 2011 Media Monitoring Desktop Reference. It was made available through the efforts of the Electronic Privacy Information Center, which filed a Freedom of Information Act (FOIA) request and subsequent law suit to obtain the documents. The manual identifies many of the problematic monitoring practices of DHS and contains a (broad) list of (extremely vague) key words DHS uses to monitor the internet (inlcuding search engines and social media communications). Unclear from the released documents is how exactly DHS is able to access the data from search engines and social media communications in its efforts to "track" the use of these keywords. However, as several reports have indicated, this is most probably achived through the agreements with host sites such as Google, Bing, Yahoo, Facebook, Twitter, and others.
The list of keywords is divided into the following categories:
DHS &Other Agencies
HAZMAT and Nuclear
Health Concern + H1N1
Southwest Border Violence
Below are the words listed within the category of "Terrorism":