The shootings took place across three separate incidents in a bowling alley, a restaurant and a Walmart distribution centre in Maine's second most populous city of Lewiston.
US President Joe Biden termed gun violence as the "ultimate superstorm" and announced the launch of a first-ever federal office on resolving issues related to shootings and support families affected by it.
The administration of President Joe Biden moved Thursday to close loopholes that allow people to sell guns without a dealer license, which would require them to perform background checks on purchasers.