1. A method comprising: classifying an initial set of source documents into one of at least three groups based at least in part on at least one classification identifier, the at least three groups comprising a first group containing documents of interest, a second group containing documents not of interest and a third group containing documents where the interest level is undetermined; and for each document classified into the third group: extracting from a document at least one feature vector; and classifying the document into either the first group or the second group based on the at least one feature vector.