The mother of a Manchester United worker who alleged he was racially abused and assaulted by security staff has said she is “surprised, disappointed and hurt” by what she described as the club’s lack of support for the teenager.

Police are investigating allegations that Kamarl Nelson, 19, was assaulted in a raciallyaggravated incident involving the club’s security guards outside Old Trafford before a Europa League match last month.

The teenager, who works cleaning and catering shifts at Old Trafford to save money for university in September, has worked at the ground for six years, including as a ball boy, and has played for the club’s youth development team.

Old Trafford guards accused of racist attack on 19-year-old worker Read more

He said he sustained a fractured eye socket and bruised ribs after he was allegedly slammed against a wall, put in a choke hold and called a “slave” by guards who disputed that he worked at the stadium.

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

The Guardian has interviewed five people who independently said they witnessed security guards surrounding Nelson in an intimidating way before the alleged assault, including one who said he heard the teenager be racially abused.

Tom Tatler, 53, said he heard one of the guards use a racial slur towards the teenager while the guard aggressively disputed that Nelson worked at the club. Tatler said: “I heard one of the security guards say, ‘You work here?’ and ‘You don’t fucking work here, and if you do, what do you do – make fucking cups of tea or clean the boots or what?’.”

Tatler said he then heard the guard racially abuse Nelson and so confronted him: “I said to the security guard it was out of order saying that … and he went: ‘Fuck off you, mind your own business’.” Tatler said he then told a nearby police officer what he had witnessed but was again told to mind his own business.

Nelson is an outsourced contractor at the stadium, employed by the recruitment agency Arc. He was wearing his uniform with Manchester United labels and pushing his bike when he arrived at the stadium on 27 February.

A season-ticket holder, Mat Shufflebotham, 48, said he could not hear the exchange but saw Nelson surrounded by three security guards, with one grabbing his bike to prevent him from getting to work.

“It was getting a bit heated but I wouldn’t say Kamarl was the aggressor – it was the other three guys,” he said. “From what I saw, it was a bit heavy handed.”

Three others – a fanzine seller, Harry Robinson, 19; Calum Young, 21; and Wendy Smith, 59 – said they saw Nelson being surrounded in an intimidating manner but could not hear the exchange.

Robinson said: “It looked like they were trying to take his bike off him. He [Nelson] didn’t seem to be doing that much wrong. He seemed a little distressed. He was by himself against six or seven [guards] so I would say they seemed to be the aggressors.”

CSG declined to comment on the five new witness accounts but repeated its denial of the allegations. The alleged incident was first made public by the community interest group Kids of Colour, which is supporting Nelson.

The teenager’s mother, Donna James, said she was “really surprised” the club had not offered her son any support, given his longstanding ties to Manchester United. He was a ball boy for years and at 15 was mentored by a coach with the Manchester United Foundation. He was pictured in a brochure, where he was quoted saying that working with the club’s charitable arm had helped him “not just progress as a player but as a person”.

James, 43, said: “I thought once [Manchester United] recognised his affiliation to the club they would have come forward and might even have come round and seen us as a family. They might have had a chat with Kamarl and offered him personally some reassurance that they were looking into it. But none of that has happened.

“That’s really disappointing because Kamarl has really fond memories of being at United from being young and doing things there. He really enjoyed his time as a ballboy there. It’s really hurtful that they haven’t put more of a human response to it.”

It is understood that club officials had spoken with Nelson’s father and grandmother to confirm they were cooperating with the investigation – contact which James said her family initiated – and that an employee of Arc had been in touch on Thursday.

A Manchester United spokesman said: “We have been in touch with the family on a number of occasions since the alleged incident and have reiterated our commitment to fully cooperating with the police investigation.

“We have provided CCTV footage and other information as part of their enquiry and would encourage any witnesses to contact the police. We await the outcome of that investigation.”

Greater Manchester police said on Friday it was examining CCTV footage from the night. No arrests have been made.