The girl was reported missing from her home on the Scottish island of Bute at 6.25am on Monday and her body was found in woodland less than three hours later.
Initially, police did not release any information about the arrested male but they later said he was aged under 18.
They continued to appeal for information.
The senior investigating officer, Detective Superintendent Stuart Houston, said The response to our earlier public appeals has been significant.
However, despite this evening’s major development, I am still appealing for anyone who was in the Ardbeg Road area of Bute on Sunday night or in the early hours of Monday and who may have information about Alesha’s death to contact us.
Anyone who has CCTV at their home or business, or indeed any motorists with dashcam footage which might help with our investigation are also urged to get in touch.
On Tuesday night, Chief Superintendent Hazel Hendren warned the community on Bute that they should be vigilant and look after each other.
A murder investigation was launched as a result of the findings of a post mortem examination into Alesha’s death.
She vanished from a home in Rothesay where she was staying and was reported missing by her grandmother at 6.25am on 2 July.
Alesha’s body was discovered by police in the grounds of an abandoned hotel at around 9am.
On Wednesday there was an increase in police activity at the property on Ardbeg Road where Alesha had been before she disappeared.
Several officers entered the garden with long sticks and searched the grounds.
Later, a recovery truck removed a black Dacia car from the rear of the house. It was the second car to be removed.
Officers have been carrying out house-to-house inquiries and conducting 24-hour high-visibility patrols to support the island’s 7,000-strong community.
Alesha was remembered by a school teacher on Wednesday as as a friendly and happy child.
Wendy Davie, headteacher of Chapelside Primary School in Airdrie, North Lanarkshire, where Alesha had just finished her second year, said she was part of the family and would be greatly missed by pupils and staff.
She loved being at school and enjoyed all aspects of literacy, in particular writing, Ms Davie said. She was such a perfectionist in her handwriting and was very proud of her work.
Alesha was very friendly and she welcomed everyone first thing in the morning at breakfast club. She was a very considerate child who loved being part of a group and she was popular with all the other children and was a smiley and happy young girl.
It is understood that Alesha had been living in North Lanarkshire with her mother Genie Lochrane, who is separated from her father, during term time.
The home from which she went missing was occupied by her paternal grandparents and Alesha’s father Robert MacPhail.
She was a few days into a three-week break.
Family and friends of the murdered girl have continued to place flowers and cards outside her home.
Police said the six-year-old’s family was devastated.
News Source SkyNews