Kai Nazir is understood to have posted the picture on Instagram on Friday
A convicted murderer has been reported to police after posting on Instagram from a mobile phone in jail.
Kai Nazir, 20, was sentenced on 10 September for stabbing Chris West to deathand attempting to kill another man at a Cambridgeshire pub last year.
Nazir posted two images and a video in which he said he was "just chilling", believed to have been on 14 September, as reported by Cambridgeshire Live.
A HMP Peterborough spokesman said the prison had taken "appropriate action".
A Cambridgeshire Police spokeswoman said: "I can confirm that as of Monday, 17 September we are aware of reports that a prisoner at HMP Peterborough has access to a mobile phone and social media from within the prison.
"We are currently looking into the circumstances."
It is understood that the pictures were posted on his Instagram page on Friday, four days after being sentenced at Cambridge Crown Court, but his account has since been deleted and the phone has been confiscated.
It is an offence to have a mobile phone in prison with the maximum penalty for possession of two years imprisonment.
Nazir, previously of St Neots, was jailed for a minimum of 26 years for killing Mr West by stabbing him 12 times outside the Monkfield Arms in Cambourne at a Halloween disco in October 2017.
He had earlier stabbed Mr West's friend, Daniel Berryman, at a pool table, causing him life-threatening injuries.
A HMP Peterborough spokesman said: "Mobile phones are a challenge across the whole prison estate. Our staff at HMP Peterborough work very hard to prevent these items getting into the prison.
"We have taken appropriate action in the prison and reported this incident to the police."