“In these attacks, known as ‘signature strikes,’ drone operators fire on people whose identities they do not know based on evidence of suspicious behavior or other ‘signatures.’ According to anonymously sourced media reports, such attacks on unidentified targets account for many, or even most, drone strikes. Despite that, the administration has never publicly spoken about signature strikes. What is the legal justification for signature strikes? What qualifies as a ‘signature’ that would prompt a deadly strike? Do those being targeted have to pose a threat to the United States? And how many civilians have been killed in such strikes?”
http://www.propublica.org/article/drone-war-doctrine-we-know-nothing-about