Federal bureaucrats are important sources of information about policy problems. However, federal officials compete for this influence with organized interests plying their own problems and solutions. We attribute the differential agenda influence of the federal bureaucracy to efforts in Congress to construct workable problem definitions in a context of uncertainty about issues. From both behavioral and rational models of congressional decision making, we develop a theory of congressional search for information during problem definition under conditions of uncertainty. The theory presages the prominence of federal bureaucrats in this search, and especially under uncertainty. Using new data sets capturing the appearance of federal bureaucrats at congressional hearings, we find that the mobilization, prominence, and types of federal bureaucrats providing information is explainable in terms of congressional uncertainty about problem definitions.