Police have launched an investigation after a teenager who admitted killing his friend posted a video of himself making stabbing motions just hours after he was found not guilty of murder.

The 17-year-old boy, who can be referred to only as Boy A, admitted fatally stabbing Yousef Makki but said it was self-defence and was cleared of murder and manslaughter.

It can now be disclosed that shortly after his acquittal the boy published a video of himself making a stabbing action and listening to drill music about “shanks” – slang for knives.

The video was then sent by an unknown person to a member of Makki’s family, causing them extreme distress while they were coming to terms with the not guilty verdicts.

Makki’s family immediately reported the incident to the judge, Justice Simon Bryan QC, and Greater Manchester police, which has now launched an investigation into the video.

Detectives are investigating whether an offence has been committed under the Malicious Communications Act by whoever sent it to Makki’s family. Lawyers for Boy A told the Guardian the video was “intended as a private message” for the teenager’s girlfriend only and “does not reflect lack of remorse on his part”.

Matthew Claughton, of Olliers solicitors firm, which represented Boy A, said: “It reflected his frustration with the way the prosecution were misrepresenting videos that were played at court. It’s worth remembering that the jury appear to have agreed with that view given the not guilty verdicts.

“It will come as scant consolation for Yousef Makki’s family but my client knows he will have to live with the consequences of that day for the rest of his life.”

Boy A was sentenced to a 16-month detention training order after admitting perverting the course of justice and possession of a bladed article. He will spend half of that sentence in custody.

The trial at Manchester crown court had heard how Makki died after a row with his friends, Boy A and Boy B, also 17, on 2 March following a botched attempt to buy £45-worth of cannabis in the Manchester suburb of Hale Barns.

Boy A was attacked, leading to a later confrontation with Makki. Boy A told the jury he pulled out a flick-knife because Makki had produced a knife – a claim denied by the victim’s family – and Boy A said he then accidentally stabbed his friend in self-defence.

Makki’s family boycotted the sentencing on Thursday in order to “make a stand” against Boy A’s acquittal. Amid chants of “Justice for Yousef”, a 200-strong crowd made up of family members, schoolmates and supporters gathered on the steps of the court to express their disappointment that no one has been found responsible for Makki’s killing.

Boy A’s barrister, Alistair Webster QC, said the teenager had expressed “genuine insight and regret” about the consequences of his actions and that he “does not seek to underplay the serious nature of his role in the death of his friend”.

Webster said Boy A had acknowledged that his sentence for carrying a knife and perverting the course of justice “will never be long enough” for Makki’s family or the wider public.

Boy B, who was not charged with murder or manslaughter, was sentenced to a four-month detention training order after he pleaded guilty to carrying a knife in an incident earlier that day. He too will spend half of that sentence in custody, and the remaining half under the supervision of youth offending teams in the community.

• This article was edited on 23 August 2019 to amend details relating to Boy B.