Investigations are increasingly conducted online by not only novice sleuths but also by professionals -- in both competitive and collaborative environments. These investigations rely on publicly available information, called open source intelligence (OSINT). However, due to their online nature, OSINT investigations often present coordination, technological, and ethical challenges. Through semi-structured interviews with 14 professional OSINT investigators from nine different organizations, we examine the social collaboration and competition patterns that underlie their investigations. Instead of purely competitive or purely collaborative social models, we find that OSINT organizations employ a combination of both, and that each has its own advantages and disadvantages. We also describe investigators' use of and challenges with existing OSINT tools. Finally, we conclude with a discussion on supporting investigators' with more appropriable tools and making investigations more social.