Computer network defense analysts engage a difficult, though critical, task in cyber defense. Anecdotally, these operators complain of frequent task interruptions while they are performing their duties. The goal for the current study was to investigate the effect of a commonly reported interruption, answering email, on accuracy and completion times in a simulated network analyst task. During task trials, participants were interrupted by emails between alert investigations, during alert investigations, or not at all (control). The results indicated that email interruptions increased alert completion times regardless of when they occurred, but interruptions that occurred during an alert investigation also reduced the accuracy of subsequent judgments about alert threat. Overall, the results suggest that task interruptions can potentially undermine cyber defense, and steps should be taken to better quantify and mitigate this threat.