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
Weather: Clouds and outbreaks of rain
Labour leader Ed Miliband has defended plans to cap tuition fees after the policy was blasted as 'incompetent'.
Ed Miliband has just announced Labour's 'plans for young people', while David Cameron has pledged to protect pensioners.