A 43-year-old man has been arrested on suspicion of the murder of a seven-year-old girl in 1992, UK police say.
Nikki Allan was found dead in a derelict Old Exchange Building near her family home in Sunderland, in northeast England, on October 8, 1992.
Police have been investigating for more than 21 years and have made a fresh appeal for information.
A spokesman said detectives were particularly keen to speak to anyone who lived in the area at the time of the murder or anyone who hung around the Old Exchange Building.
Police worked with the BBC Crimewatch show in September to make a reconstruction of Nikki's last movements.
The arrested man was not George Heron, who was cleared of her murder after a trial in 1993, a source said.
Mr Heron was acquitted despite apparently making a confession after a judge ruled he had been dealt with oppressively by police.
Nikki's mother Sharon Henderson subsequently won a civil claim against him, and he was ordered to pay her 7,000.
Nikki vanished after going out to collect pennies for Guy Fawkes night.
Forensics showed she had been stabbed 36 times and she was beaten over the head with a brick.
Before the Crimewatch show was broadcast, Ms Henderson, who has three other daughters, said she had never given up hope "that justice will be done".
"I have fought for so long and I know Nikki would be proud of me."
