Two postcards may hold the key to solving a decades-old unsolved killing of a U.S. mother-of-two who was last seen alive heading off to spend her day sketching and taking photographs, according to authorities in England.

Shelley Morgan, 33, was last seen alive at 8:30 a.m. on June 11, 1984, after dropping her children off at school before heading towards the Leigh Woods area in the city of Bristol. Her remains were discovered five months later in the woods by a group of children.

“It’s impossible to imagine the pain and anguish Shelley’s family have felt over the years and understand the impact her murder has had on her two children, who’ve been left without a mother for the majority of their lives," said Det. Sgt. Sarah Barnston of the Avon and Somerset Police Department's Major Crime Review Team.

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Morgan, who was originally from the U.S., had been stabbed multiple times and "there was evidence the brutal attack had been sexually motivated," according to police.

Avon and Somerset Police said in a news release on Monday that the department's major crime review team has revealed two postcards that may "have a significant bearing" on their investigation.

The postcards, which depict scenes of the River Avon and St. Andrew's Church, were from a calendar sold by the local Bristol Hospice charity in the 1980s or 1990s.

"Both locations are significant as they are linked to the areas where Shelley was heading for on the day she disappeared, and where her body was found five months later," police said.

Authorities are looking to speak to anyone who may have bought the calendar or may have kept their tear-off postcards with the specific images, "possibly for some time."

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Barnston said Morgan was "passionately interested" in art and theater while taking art classes at the time of her murder at Bristol Polytechnic Art College.

The 33-year-old's work and locations where she was planning on visiting the day she went missing may "hold the key to her killer’s identity," according to Barnston.

"We remain of the belief that her killer must have had access to a vehicle and may have had links through work or other associations to Backwell and possibly the Leigh Woods or Clifton areas of Bristol," she said. "Although there were numerous sightings of Shelley on the day she went missing, in both the Bristol and Backwell areas, none have been confirmed."

Camera equipment belonging to Morgan has also never been found, including an Olympus OM20 camera, a tripod, and sketching materials in a large patchwork-style shoulder bag. The missing 35mm camera with serial number 1032853 is still popular with collectors and photography students today, according to Barnston.

“I would urge anyone with information, no matter how small, to come forward," Barnston said. "Loyalties and relationships change over time and there may be someone who didn’t feel able to speak to us at the time, but who may be in a position to do so now."

In a video message marking the 35th anniversary of her murder, Morgan's sister Hollie Brian made an appeal for anyone with information to come forward to authorities.

"I've come to believe that people aren't really completely dead until the people who love and remember them have passed also," Brian said. "As long as we carry Shelley in our hearts she will always be with us, but we are getting older and the time will come when all those doors are closed."

Brian said Shelly left behind her husband and two small children, one of whom is autistic. She said if anyone was afraid to speak out in 1984, maybe their situation has since changed and they can contact police.

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"Out of love and respect to my sister we really want to know what happened and be sure that the person that killed her will never hurt anybody ever again," she said.