The weapon that killed Agent David Maland last week in northern Vermont was bought by a person of interest in a double murder two years ago in Delaware County, state police say.

As the sprawling investigation unfolds, here’s an overview of the key figures involved and their alleged connections.

The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives sent an alert to federally licensed firearms dealers a day after the shooting seeking information related to any gun purchases by Michelle Jacqueline Zajko.