A man accused of killing a nine-year-old boy by firing at passing cars told police he did it because people had been waking him up by revving their engines.
Nhan Lap Tran, 34, of St Paul, Minnesota, is charged with six offences in the February 11 rampage.
Prosecutors say he had previously admitted the shootings but had not given a motive.
The St Paul Pioneer Press reports that police found a note in his bedroom when they arrested him after the shootings that read: "Random Kill, Fake Plates."
An affidavit filed in Washington County District Court says the date "12/12/12" was also scrawled on the bedroom walls.