Prosecutors have said the 17-year-old told several students about his plans to shoot up South Portland High School and asked at least one other student to help.
Police fatally shot Christapher Dodge, 47, early Friday morning after responding to a domestic violence shooting at the Summit Terrace apartment complex.
New DNA tests failed to link Dennis Dechaine to the murder of 12-year-old Sarah Cherry, but the Maine Attorney General said other evidence supports his conviction.