Web-based data analysis environments are powerful platforms for exploring large data sets. To ensure that these environments meet the needs of analysts, a human-centered perspective is needed. Interfaces to these platforms should provide flexible search, support user-generated content, and enable collaboration. We report on our efforts to design and develop a web interface for a custom analytics platform-EPIC Analyze-which provides interactive search over large Twitter data sets collected during crisis events. We performed seven think-aloud sessions with researchers who regularly analyze crisis data sets and compiled their feedback. They identified a need for a "big picture" view of an event, flexible exporting capabilities, and user-defined coding schemes. Adding these features allowed EPIC Analyze to meet the needs of these analysts and enable exploratory research on crisis data.