13 year-old Casey Kearney died on Valentines' Day after she was stabbed in Elmfield Park in Doncaster.
Police want people who were seen near the park just after 1pm that day to contact them.
More top news
Many places dry and bright into Sunday but feeling cooler and winds stronger after overnight rain clears
Dry, sunny start with cloud forming later on
Hull could be wiped off the map within 100 years because of coastal erosion, according to an expert