(ABC 6 News) — The suspected killer of UnitedHealthcare CEO Brian Thompson, Luigi Mangione, remains behind bars in Pennsylvania.

This comes while Manhattan prosecutors present evidence to a grand jury in hopes of an indictment.

Sources say police are working to piece together the suspect’s movements. Investigators now believe Mangione took the subway to Penn Station and then a train to Philadelphia, walking back the idea that the gunman hopped on a bus.

Investigators are also looking into whether UnitedHealthcare was targeted because of its size.

UnitedHealth Group CEO Andrew Witty admits the healthcare system is flawed, writing in a New York Times essay that “no one would design a system like the one we have, and no one did. It’s a patchwork built over decades. Our mission is to help make it work better.”