A coalition of church leaders, lawyers and activists in North Carolina have called on federal authorities to aid in the investigation into the mysterious death of Lennon Lacy, an African American teenager found hanging in August from a child’s swing set in the middle of a largely white trailer park.

The North Carolina branch of the National Association for the Advancement of Colored People (NAACP) spent almost two hours on Tuesday with US attorney Thomas Walker in his Raleigh office. The NAACP handed the federal prosecutor a letter formally asking the FBI to join the investigation. The association also presented the prosecutor with a file of more than 20 leads that it believes casts doubt on the suggestion that the 17-year-old high school football player killed himself.

The letter questions the “quick call” reached by local police officers that Lacy had taken his own life entirely alone, with no involvement of anybody else or any foul play. “Given evidence uncovered by the (North Carolina) NAACP, there are several other possible explanations for his death besides a simple suicide,” the letter says.

The request that the federal government in effect take over the investigation into Lacy’s death was released to the public at a press conference at which the teenager’s mother, Claudia, spoke of her anger about the inquiry so far. “I just want to know what happened to my 17-year-old son; all I want is justice,” she said.

She added that she felt she had been treated by agents of the North Carolina state bureau of investigation as though she had herself been a suspect. “I feel the SBI investigators interrogated me. They were not trying to find out the truth of what happened to my son, they were pushing towards a verdict of suicide.”

The NAACP has called on the US attorney to bring the feds into the investigation citing the Department of Justice’s powers under the Hate Crimes Prevention Act, the Conspiracy Against Rights Act and the Violent Crime Control and Law Enforcement Act. Walker’s office in the eastern district of North Carolina declined to comment on the invitation or whether it would be taken up.

Michael Discioarro, a former New York prosecutor, told the Guardian that where a federal attorney believed a victim’s rights had been breached, they had the ability to act. “They can do pretty much what they like – they can start an investigation, get an indictment and launch a prosecution.”

Lacy’s death on 29 August has spread a chill across the small town of Bladenboro. The image of a black male found hanging from a beam is gruesomely resonant across the south, given the history of lynchings.

He was found hanging just a short walk away from his home hours before he was set to appear in a high school football game for which he had been training for months. Detectives appear to have latched on to the idea that he was depressed a day before he died. Lacy’s uncle, with whom he was close, had been buried - though the family disputes there were any signs of depression beyond normal sadness. Five days after his death, the local investigating team announced that no evidence of foul play had been found, implying a probably suicide.

Claudia Lacy and Lennon’s brother Pierre were unsatisfied by what they considered to be a rush to judgment. They called on the NAACP to advise them, and in turn the organization uncovered what it considered to be a failure to treat the inquiry seriously.

Reverend William Barber, president of the North Carolina branch, told the press conference that “if the basic facts were reversed, and Lennon was white and found hanging in a predominantly black neighbourhood, would there have been such a rush to quickly say this was a suicide?” He declined to give specific details, saying that the NAACP was determined not to muddy the investigation, but added that evidence had been found suggesting that racial tensions played a part.

Lennon Lacy was in a relationship with a white woman who lived over the road from him at the time, or shortly before, he died. “He did have an interracial relationship and attended an interracial church and people in that community raised their dislike of that,” Barber said. The NAACP letter to Walker also refers to a possible “race-based animus toward Lennon and his family by some of their neighbours”.

The NAACP has deployed a pathologist to carry out a separate study of the autopsy that was conducted. One aspect that has been discounted was the initial impression that Lacy’s body had scratches and contortions on his face – those marks appear to have been caused by ant bites or through handling of the body after the postmortem had been concluded.

The association has also employed a lawyer specialising in forensics to take a second look at the way the police investigation was conducted. The lawyer, Heather Rattelade, said that her most intense concerns were over how the town’s investigators backed up by state agents had staged the initial crime scene investigation.

Rattelade said that at 3.08pm on the day of Lacy’s death – possibly just nine hours after his body was discovered at the swing set – the lead investigator told the medical examiner that the police believed they were dealing with a suicide. Yet they had failed to meet “even the minimum standards for crime scenes. I cannot tell you why minimum protocols were not followed, but that is a large part of the reason why we have been compelled to ask the department of justice and the FBI to carry out its own investigation into what happened to Lennon Lacy.”