Teenage killer Jay Stewart has posted a video online from jail in which he appears to boast about stabbing his victim Kayden Moy.

Stewart, 19, and Cole Turley, 18, pleaded guilty to the murder on Irvine Beach in May last year, and were given life sentences last week.

The video, filmed in a cell in HMP Polmont and posted on Snapchat, shows Stewart topless, masked and rapping about the killing – claiming he will kill again once he is released.

The BBC understands the Scottish Prison Service is investigating the matter after it was reported to them anonymously.

Stewart and Cole Turley, 18, who had also pleaded guilty to the murder, were told they will serve a minimum of 16 years in jail.

A third teenager – a 15-year-old who cannot be named for legal reasons – will be detained for a minimum of 14 years.

In the expletive-laden video, Stewart refers to apprentice painter and decorator Kayden repeatedly, including by the initials KM – he says that people know “KM is dead”.

He says: “Everyone knows my scheme is violent, big plan for future ahead. I just love being a bloody ned.”

He also appears to threaten further acts of violence “when I get out”.

A court heard that Stewart and the 15-year-old – while acting with Turley – chased Kayden, from East Kilbride, and repeatedly stabbed him with a lock knife after he fell to the ground.

Stewart brandished his baton and was heard to shout “Murray Boys”. He then told a witness at the scene that his victim “deserved it.”

After running from the scene, Turley stuck the knife into the grass and was overheard boasting that he “just done him.”

Turley also shouted “One-nil.”

CCTV showed the trio running down a road near the beach – stopping momentarily to embrace each other.

After the three were sentenced, Kayden’s father Paul Moy said his family’s grief meant they were themselves enduring a life sentence.

He said though the outcome brought a “measure of justice”, it did not bring his son back – and he hoped the sentences sent “a clear message that violence has a devastating and irreversible consequences”.

The Moy family have since seen Stewart’s video – Paul Moy posted online he was “devastated” to see the killer “singing, laughing, and appearing to boast about taking my son’s life”.

He called on the prison service to carry out a full investigation.

He said: “Nothing will ever prepare a parent for something like this. We live every day with the heartbreak of losing Kayden while the person responsible appears to show no remorse and no respect .

“This raises serious questions. How was this allowed to happen? How was a video like this recorded and leaked from inside a prison? Victims’ families deserve answers, and we deserve better than to have our grief reopened in such a cruel and unnecessary way.”

A Scottish Prison Service spokesperson said: “We understand the lasting impact this kind of offending has on families and the significant distress such videos can cause.

“Our staff work hard to keep prisons safe and secure, using all available technology and intelligence to prevent illicit items, including mobile phones, entering our establishments.

“While we do not comment on individuals, we continue to work with Police Scotland, and other partners, to take action against those who attempt to breach our security.”

The BBC has approached Police Scotland for comment.