RCMP Sgt. Barb Thornborough says the man witnessed the fatal hit-and-run crash Wednesday afternoon and scratched the information on the sidewalk before leaving.
The RCMP say an elderly couple was hit by a grey truck making a turn onto a local road.
Seventy-six-year-old Juan Jose Rosales and his 66-year-old wife Marta Dolores Rosales were both killed.
Police say the truck driver fled the scene, but officers later arrested a male suspect and located a vehicle.
Drugs and alcohol have been ruled out as factors in the incident, and anyone with information is asked to contact the police.