Insider threat detection and investigation are major challenges in digital forensics. Unlike external attackers, insiders have privileges to access resources in their organizations and violations of normal behavior are difficult to detect.This chapter describes an unsupervised deep learning framework for detecting insider threats by analyzing system log files. A typical deep neural network can capture normal behavior patterns, but not insider threat behavior patterns because of the presence of small, if any, amounts of insider threat data. For example, the autoencoder unsupervised deep learning model, which is widely used for anomaly detection, requires a dataset containing labeled normal data for training purposes and does not work well when the training dataset contains anomalies. In contrast, the framework proposed in this chapter leverages unsupervised multi-autoencoder filtering to remove anomalies from a training dataset and uses the resulting trained Gaussian mixture model to estimate the distributions of encoded and recognized normal data; data with lower probabilities is identified as insider threat data by the trained model. Experiments demonstrate that the multi-autoencoder-filtered unsupervised learning framework has superior detection performance compared with state-of-the-art baseline models.