Retired nurse Jenna Sansone said it was a normal night in the quiet eastern Iowa neighbourhood where she lives on a hill above the Mississippi River. Then she heard the “bang-bang-bang” of gunshots and called 911.

Responding officers made a gruesome discovery: the body of a teenager dumped in the alley behind Sansone's Burlington home, a plastic garbage bag wrapped around the victim's head. Sansone could smell the bleach that had been poured on the bullet-riddled body, an apparent attempt to destroy DNA evidence.

The March 2016 death of 16-year-old Kedarie Johnson stunned the city of 25,000. It also raised fears that the popular Burlington High School student was the victim of a hate crime because the teenager was gender-fluid — identifying both as male and female, according to relatives, and alternately using his birth name and the female persona “Kandicee.”

On Tuesday, one of two cousins charged with first-degree murder in Johnson's death is expected to stand trial in the case, which Attorney General Jeff Sessions has highlighted with his unusual decision this month to send a federal hate crimes prosecutor to assist state and local authorities. A decision on federal hate crimes charges is pending, but the prosecutor assigned by Sessions indicated in court documents that Johnson's gender identity played a role, with the cousins attacking the teenager after discovering during a sexual encounter that Johnson was gender nonconforming.

The trial of 23-year-old Jorge Sanders-Galvez, an aspiring rapper who went by the nickname “Lumni,” is expected to last two weeks and could clear up “a lot of questions about what happened and why it happened,” said Sansone, who plans to attend at the request of Johnson's mother.

The prosecutor assigned by Sessions, Christopher Perras, wasted no time in helping provide the public some long-sought answers. In a court document filed on Wednesday, Perras detailed the prosecution's theory of the killing for the first time.

Perras wrote that evidence will show that Sanders-Galvez began flirting with Johnson after seeing the teenager leave a Hy-Vee store. Sanders-Galvez persuaded Johnson to get in his car, then brought Johnson to a home to have sex with him and his cousin, 25-year-old Jaron Purham, Perras wrote.

Sanders-Galvez became enraged during the encounter after discovering Johnson was “biologically male” and “over the next hour suffocated Kedarie by stuffing a rag down his throat and wrapping a plastic bag around his head, threw him in their car, drove to another part of town, dumped him in an alley and shot him several times until he bled to death,” Perras wrote. An autopsy found Johnson died from gunshots to the chest.

Sanders-Galvez has pleaded not guilty and told investigators last year that he had never met Johnson. He faces life in prison if convicted. His attorney declined comment.

The federal government's intervention has been welcomed by LGBTQ-rights advocates who have criticised Sessions and President Donald Trump for broadly eroding their legal protections.

The allegations are horrific and highlight how Iowa law, unlike federal law, doesn't allow prosecutors to bring hate crime charges based on gender identity, said Daniel Hoffman-Zinnel, executive director of One Iowa, an LBGTQ advocacy group.

“These things are horrible and very unfortunate but they force us as community members to have these difficult conversations and hopefully we can work to prevent them from happening,” Hoffman-Zinnel said.

Purham, who is expected to stand trial later, is serving time in Missouri after he was convicted on charges related to his flight from officers tying to arrest him last year. Officers said they recovered a .357 revolver in Purham's vehicle after he drove into a police car outside St. Louis, and testing showed that it was the gun used to shoot Johnson.

A federal grand jury has been convened to consider whether the men should face hate crime charges, but no decision has been announced.

It's unclear whether Johnson and Sanders-Galvez had previously met, but investigators said the two were friends on Facebook.

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Investigators said that before the slaying, Johnson had stopped at a female friend's home, telling her he was scared of someone named “Lumni” and worried he was being followed by a red car. Johnson asked to borrow several bras, which he put in his backpack before leaving alone and on foot, according to court documents.

Investigators said Johnson is seen on surveillance video outside the Hy-Vee store wearing braids, pyjama bottoms and a black jacket as a red impala linked to Purham and Sanders-Galvez follows closely behind.

According to investigators, they later found Johnson's backpack at a Burlington home where Purham and Sanders-Galvez were staying with Purham's pregnant girlfriend in the weeks before the killing. Both men fled to St. Louis area within 48 hours after Johnson's slaying, they said.