The incident occurred in the state of North Carolina, after the storm broke a tree which has slammed into a car, killing the passengers, the NBC broadcaster reported.
Alberto is the first named storm of the 2018 hurricane season. It formed in the Gulf of Mexico on Friday and threatens to batter the Gulf of Mexico coast with heavy rains and storm surge.
A number of US states such Alabama and Florida, declared the state of emergency over Alberto last week, while over 5,000 people were evacuated in Cuba on Monday.