TRENTON, N.J. — It started with a party invitation to a 15-year-old girl from some men she knew. She took her 7-year-old stepsister to an apartment down the street from their home near the Statehouse, where the girls had been hanging around outside on a Sunday afternoon.

For the younger girl, police say it quickly descended into a horrifying ordeal in which she was gang-raped by as many as seven men as her sister not only watched but got paid by those who did it.

Their parents, none the wiser, thought maybe they had run away.

“We’re talking about a kid who told her sister to go into an apartment and let people rape her,” said Trenton police Capt. Joseph Juniak. “It’s unfathomable.”

The teen has been charged with aggravated sexual assault, promoting prostitution and other crimes. Her name was not released because of her age, but the county prosecutor plans to ask the court to try her as an adult. In the meantime, she is being held at the Mercer County Youth Detention Center.

The 7-year-old had wanted to tag along because she was worried about the 15-year-old’s safety, Mayor Doug Palmer said.

When the girls didn’t return home by 4:30 Sunday afternoon, their parents called police, believing the older one had run away from home and taken her younger sister with her.

In fact, they were down the street inside a 13th-floor apartment at Rowan Towers, a high-rise complex so dangerous that police are hired as security guards at night.

Inside apartment 13-C, police said, the 7-year-old was soon left alone as her sister headed to a back bedroom to sell sex to several men. When she came out into the living room, she handed her 7-year-old sister money and encouraged her to let the men touch her.

“It went from touching to straight out assault and rape,” Juniak said. “They threatened to kill her if she screamed or told anyone.”

Afterward, the child put on her clothes and left. Her sister stayed behind with the men.

Two women found the child crying outside the apartment and walked her home, where police were waiting.

The child told them what happened and was treated at a hospital. When police located the 15-year-old later that night, she also told them what happened and was arrested.

Palmer said the crimes are among the worst he’s seen in 20 years as mayor.

“It’s sickening,” he said. “The police are taking this personal. I know there’s a place in hell for all the people that participated in this and I’m sure they will get there. Personally, as a father with a 7-year-old daughter, I can’t imagine the horror.”