Olic Urge for help identifying a man after the fatal stabbing of nine-year-old Lilia Valutyte.
Lincolnshire Police released four CCTV images of the man and warned anyone who sees him not to approach him but to call 999 immediately.
Police added that two people arrested earlier this week have now been released without further action.
Chief Superintendent Martyn Parker said: “Our officers have been working around the clock on this investigation and the help of the public could prove crucial.”
Speaking of the CCTV footage, he said: “These are very clear images of a man we would like to speak to, so I would urge anyone with information to contact us.
“If you see him, don’t approach him, call 999 right away.
“This is a very tragic case and I want to reassure the public that we have an enormous amount of resources devoted to the investigation.
“We continue to support Lilia’s family as they mourn this unimaginable loss.”
Lilia was found on Fountain Lane in Boston Thursday around 6:20 pm.
Anyone with information should call 101 citing Incident 419 of July 28.