Although storage in working memory (WM) can be tracked via measurements of ongoing neural activity, past work has shown that observers can maintain access to that information despite temporary interruptions of those neural patterns. This observation has been regarded as evidence for a neurally silent form of WM storage. Alternatively, however, unattended information could be retrieved from episodic long-term memory (eLTM) rather than being maintained in WM during the activity-silent period. Here, we tested between these possibilities by examining whether WM performance showed evidence of proactive interference (PI)-a hallmark of retrieval from eLTM-following such interruptions. Participants remembered the colors (Experiments 1-3) or locations (Experiment 4) of serially presented objects. We found PI for set sizes larger than 4, but not for smaller set sizes, suggesting that eLTM may have supported performance when WM capacity was exceeded. Critically, performance with small set sizes remained resistant to PI, even following prolonged interruptions by a challenging distractor task. Thus, we found evidence for PI-resistant memories that were maintained across likely interruptions of storage-related neural activity, an empirical pattern that implies activity-silent storage in WM.