Armstrong, B.C.'s Halloween slaying investigation has resulted in upgraded charges to first-degree murder against 26-year-old Matthew Stephen Foerster.

Taylor Van Diest, 17, was found badly beaten on Oct. 31 last year while clad in her zombie costume. She was left unconscious near a set of train tracks, and died later in hospital.

Foerster was originally charged with second-degree murder after police arrested two suspects. He is also charged with sexual assault and unlawful confinement in connection with an attack on a female employee at a Kelowna escort agency.

His father, Stephen Roy Foerster, is charged with obstruction of justice and accessory after the fact.

Mounties say they're still looking to map out Foerster's activities between December 2011 until March this year, and have released images to advance the investigation.

Those with tips are asked by police to call 1-888-688-4264.