This project investigates political astroturfing, i.e. hidden propaganda by powerful political actors aimed at mimicking grassroots activity, on social media. We focus on Twitter accounts used by the South Korean secret service to influence the 2012 presidential elections in favor of the eventual winner, Park Geun-hye. Two independent cluster analyses based on activity patterns of the Twitter accounts and textual features of tweets reliably reveal that there are three groups of NIS accounts, including one group that engages mostly in retweeting, and another group focused on posting news articles with a link. We show that these groups reflect different strategic agendas and correspond to several secret service agents identified in the court documents. We argue that these patterns of coordinated tweeting are consistent with predictions derived from principal-agent theory, and should therefore appear in other astroturfing campaigns as well.