COLAB is an analysis environment in which multiple human analysts in different physical locations can collaborate to build hypotheses of un-folding scenarios. COLAB consists of two components: an instrumented analysis working environment built on a blackboard system, and a web-based user interface that integrates the Trellis hypothesis authoring and management tool with a query language. On the blackboard, analysts collect data and perform analyses using relational database queries and a set of analysis tools that include group identification and suspicion scoring algorithms. Evidence extracted from data analysis on the blackboard may then be incorporated in hierarchical argument structures in the Trellis tool, combining evidence sources with user-supplied free text. Trellis arguments may then be shared between analysts and collaboratively authored. COLAB has been integrated with the Hats Simulator challenge domain and serves as a prototype mixed-initiative information fusion system.