Cold Case detectives investigating the brutal murder of a 6-year-old boy that shocked England 21 years ago say they are “closing in” on those responsible.

Rikki Neave disappeared Nov. 28, 1994, as he walked to school from his home in Peterborough, a city 75 miles north of London. His naked body was found the next day in nearby woods. He had been strangled.

His mother was charged with Rikki’s murder, but she was acquitted in 1996, the Shropshire Star reports. Ruth Neave eventually pleaded guilty to abusing Rikki and was sentenced to seven years in prison. After her release, she pressed police to re-open the case, which they did earlier this year.

As the 21st anniversary of the boy’s disappearance approached, Detective Chief Richard Wall told the Star there had been “significant progress” in identifying who killed him.

Detectives also canvassed Rikki’s neighborhood Friday seeking information.

“Even 21 years on, people still remember clearly the events leading up to Rikki’s death and I would urge people to continue to come to us with any information they have,” Wall told the Star.

He added: “We are confident that we are closing in on those responsible for Rikki’s death and it is only a matter of time before we have that vital breakthrough that leads us to them.”

BBC’s Crimewatch re-constructed the crime in a feature that aired a couple of weeks ago with an appeal for information from Rikki’s sister, according to the Mirror.

The paper also reported that police recently released a sketch of two teenagers they say were seen around the crime scene at the time of Rikki’s death.