An alleged racist assault against a teenage Manchester United worker by security staff outside Old Trafford was not captured by CCTV as the cameras were pointing in the wrong direction, his family have been told.

Kamarl Nelson, 19, was allegedly slammed against a wall and put in a choke hold in what is said to have been a racially aggravated incident involving the club’s security guards before a Europa League match last month.

The student, who once played for Manchester United’s youth development team, said he sustained a fractured eye socket and bruised ribs. He said he was called a “slave” by guards who disputed that he worked at the stadium. One searched his bag and allegedly said he was “surprised I didn’t find a knife”.

Greater Manchester police launched an investigation last week and have yet to make any arrests.

Nelson’s mother, Donna James, said her son had been told that officers had scoured eight hours of CCTV and the alleged assault was not filmed as the camera was not pointing in that direction at the time.

James, 43, said officers had found CCTV of Nelson arriving at Old Trafford and making his way to security, but the subsequent confrontation appeared to have been missed.

“They [police] said that because the camera isn’t static it moves around so it’s not picked up the incident, which sounds odd – this is outside the front of Old Trafford. It doesn’t sound right at all.”

She added: “As a family we just want justice to be served. We wouldn’t like this to happen to someone [else’s] son or daughter.”

Kids of Colour, the community interest group supporting Nelson, said police initially told the teenager on 8 March – a week after the incident – that there was no CCTV available.

Days later, officers told the family they had recovered eight hours’ worth of footage from Manchester United’s cameras and were examining it.

Roxy Legane, who founded Kids of Colour, said: “This appears to be another case of racism not being addressed with the urgency and care that it requires, and that reflects on all the institutions involved and their commitment to being anti-racist.”

Nelson has been working cleaning and catering shifts at Old Trafford to save money to go to university in September. He has worked at the ground for six years, including as a ball boy.

At 15 he was mentored by a coach with the Manchester United Foundation. He was pictured in a brochure and quoted as saying that working with the club’s charitable arm had helped him “not just progress as a player but as a person”.

Controlled Solutions Group (CSG), which is contracted to run security operations for Manchester United, has strenuously denied the allegations against its employees.

Last week five people independently told the Guardian they witnessed security guards surrounding Nelson in an intimidating way before the alleged assault. One of the five said he heard the teenager being racially abused.

Greater Manchester police said enquiries were ongoing and they were attempting to trace witnesses.