A nine-year-old was stabbed to death as she played with a toy pushchair near her home in Boston on Thursday evening.
Lincolnshire police said on Friday morning that two people had been arrested in connection with the girl’s death.
Chief Superintendent Kate Anderson, of Lincolnshire Police said: "We believe this to be an isolated incident. We are pursuing extensive lines of enquiry and have committed a number of resources to this investigation."
"You will see more officers patrolling the area, and we have a high number of specialist detectives who have been brought in to assist the investigation."
"We are not yet in a position to name the victim. Her parents are being supported by specially trained officers at this very difficult time, and I can only imagine the grief and pain that they are going through. Their welfare and the welfare of those who she knew is a priority for us."
Chief Supt Anderson added: "This incident has shocked our community in Boston, and I would like to reassure the community that we’re doing absolutely everything we can to progress the investigation and to support the family."
The nine-year-old died from a suspected stab wound at around 6.20pm, sparking a major hunt for her killer.
It is understood that one lead being pursued by police is a report of a man seen running away from the scene of the attack, with officers examining CCTV for any signs of suspicious activity.
The little girl was last seen playing in the cobbled street where the attack took place with her younger sister.
One local woman said: "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."
The woman added: "I didn't see what happened, but we were told they were looking for footage of a man running away."
One resident reported seeing a young child's doll pushchair lying in the road next to paramedic equipment being used in a vain attempt to save the little girl’s life.
Another local added: "This shouldn't have happened. A parent's worst nightmare."
Distraught residents who gathered at the scene on Friday to lay tributes, described the killing as one of the worst incidents to happen in the town in recent years.
One bouquet of red roses laid at the scene carried a condolence card which read: "So sorry this cruel world has taken your life, beautiful girl. May you be in the arms of the angels."
A local described the girl as "very quiet," and said the family had done no harm to anyone.
A stretch of the area around Fountain Lane, where the girl was found, was cordoned off for forensic examination.
Two forensic officers in white suits could be seen working at the scene before leaving in the early hours of Friday morning.
A large object appeared to be covered in the middle of the alleyway, which was marked by small yellow signs.
Police are understood to be examining CCTV footage from local businesses in the area around the murder scene.
The victim's parents have been informed, and the force said their "thoughts are with them at this incredibly difficult time".
"Her family will be supported by specialist trained officers," a short statement added.
The Conservative MP for Boston and Skegness, Matt Warman, said he had been in contact with the policing minister to ensure national assistance is available to the murder investigation.
He said: "Profoundly shocking news in Boston this evening. I have spoken to senior officers locally and am hugely grateful for all the emergency services' work.
"I have also been in contact with policing minister Tom Pursglove to ensure that any national help that might be useful is made available as quickly as possible."
Fountain Lane in Boston, a market town on the east coast of England in Lincolnshire, borders the back of an old Grade II listed building called The Sessions House.
Behind the old court building on Fountain Lane lies a car park and a few boarded-up shops.
One end of the lane appears to end on a residential street, with a few shops dotted between homes. The other end leads to what appears to be a busy high street, with shops and restaurants.
Home Office figures in 2016 found that the port town has the highest numbers of murders and attempted murders per person in England.
Data showed that there were two homicides and eight attempted murders in Boston for the period up to September 2015, meaning there was a rate of 15 murderous crimes for every 100,000 people.
This was found to be the highest per person rate in England and Wales for the period up to September 2015.
The overall crime rate in Boston in 2021 was 100 crimes per 1,000 people, making it the most dangerous medium-sized town in Lincolnshire.
The crime rate came in 25 per cent higher in the market town than the Lincolnshire overall crime rate of 75 per 1,000 residents.
Boston is ranked the 55th most dangerous medium-sized town in England, Wales and Northern Ireland as a whole.
