Sensor networks are playing a more and more important role in monitoring problems such as surveillance and tracking moving objects. In-network processing has been shown to improve scalability, prolong sensor network lifetimes, and diminish computational demands. However, securely querying past data becomes a challenge. An unimportant event in the past could become interesting later; therefore, methods for securely placing a query on a past event should be available. This paper focuses on the challenges associated with securing queries on past data in a sensor network, and proposes methods for past data aggregation using in-network processing. Two different methods are proposed for securing queries on past data; both use forward secure cryptography and provide forward secure data authentication. We compare their security and performance, and also compare them with previous schemes.