Police are urgently appealing for help to identify a man following the fatal stabbing of nine-year-old Lilia Valutyte, reportedly in front of her little sister.
Lincolnshire Police released four CCTV images of the man and warned anyone who sees him not to approach him but to call 999 immediately.
The force added that two people who were arrested earlier this week have now been released with no further action.
Chief Superintendent Martyn Parker said: “Our officers have been working round the clock on this investigation, and the public’s help may prove crucial.”
Speaking about the CCTV images, he said: “These are very clear images of a man we want to speak to, so I’d urge anyone with information to get in touch with us.
“If you see him, do not approach him but immediately call 999.
“This is a particularly tragic case and I’d like to reassure the public that we have a huge amount of resources dedicated to the investigation.
“We continue to provide support to the family of Lilia, as they grieve following this unimaginable loss.”
Lilia was found on Fountain Lane in Boston at around 6.20pm on Thursday.
She had reportedly been playing with her younger sister on the street just yards from an office where their mother was working at the time.
The girls had a hula-hoop, and a toy pram was left at the scene, according to local people.
A resident from the area told the Telegraph: “The little girl was playing in the street with her younger sister when it happened. They had some toys out.
“I’m sure her mum wouldn’t have been far away as she always checked on them. She was a very quiet girl, and I just can’t understand it.
“I didn’t see what happened, but we were told they (police) were looking for footage of a man running away.”
Jurate Matulioniene, a family friend and chairwoman of the Boston Lithuanian Community, described Lilia as “a beautiful angel” who loved dancing.
She told The Times: “Lilia was a beautiful angel, she was quite quiet and loved dancing.
“The family used to come to all of our community events and they would always stay late to help clear up.
“I have spoken to the mum and asked for her account details because we want to do a fundraiser but she would not accept it.”
Anyone with information should call 101 quoting Incident 419 of July 28.