Police are investigating a mass stabbing where a 42-year-old man entered the Walmart in Traverse City and used a folding knife to stab 11 people.

The authorities on Sunday said that a motive for the attack, which left 11 people injured, remained unknown. Citizens, including one who was armed, helped to apprehend the suspect, officials said.

Eleven people were taken to Munson Medical Center in Traverse City after a man stabbed shoppers in a nearby Walmart store.