A copy of an Adolf Hitler speech has been found at the home of a 23-year-old white suspect accused of killing two black men and firing on a family, in a string of attacks that police say may have been racially motivated.

The killings of two black men in the US state of Louisiana last week were probably racially motivated, police said, and a 23-year-old white man is in custody.

Sergeant L'Jean McKneely said the shooting happened on Tuesday night while the two children were alone in a car parked at a shopping centre.