In this study, we aim to reveal patterns of e-petition co-signing behavior that are indicative of political mobilization of online “communities” in the case of We the People (WtP), the first web-enabled petitioning system for the US federal government. This Internet-based tool enables users to petition the Obama Administration and solicit support for policy suggestions. Using WtP petition data, we focused on 33 petitions that were initiated the week after the Sandy Hook shooting (December 14-21, 2012) involving gun control and collectively received a response from President Obama. We apply Baumgartner and Jones’s (1993) work on agenda setting and punctuated equilibrium, which suggests that policy issues may lie dormant until a “focusing event” triggers the attention from political figures, interest groups, and the media. Using market basket analysis and social network analysis we found evidence of the mobilization of online communities for and against gun control laws and alternative policy proposals to address the Sandy Hook tragedy.