A registered doctor administered a full anaesthetic to the teenager in a private hospital in Egypt's coastal Suez province, according to Dr Lotfi Abdel-Samee, the health ministry undersecretary in the province, and then began surgically removing part of her sexual organs.
Mousa did not survive.
She died of complications Sunday caused by female circumcision, Abdel-Samee told CNN. It is a common procedure in the region known as female genital mutilation (FGM). The practice has been illegal in Egypt since 2008 but remains a strong tradition in Egyptian society where families see cutting as a way to "calm" or "purify" young girls.
The initial results of an autopsy performed on the girl cite blood clotting as the possible cause of death, according to Abdel-Samee. El Canal National Hospital, the facility that carried out the banned procedure, was shut down by authorities after Mousa's death and the case is currently under investigation by state prosecutors, Abdel-Samee said.
"We have been vigilant in monitoring for these cases and we have increased our focus. But we have not seen anything like this in recent months," Abdel-Samee said. "This is a very dangerous and bad practice that must end."
Ninety-two percent of married Egyptian women aged 15 to 49 have been subjected to FGM, according to a recent government report. Even more alarmingly, 82% of female circumcisions in Egypt are performed by trained medical personnel, the United Nations reports.
Mousa's mother, who is a nurse at the same hospital, denied to authorities that her daughter underwent FGM and says she was at the hospital for a different operation, according to Abdel-Samee. The incident was discovered by a health inspector who was called to check on the girl's death and found, after closer evaluation, that medical staff had illegally preformed FGM.
"This is part of the tragedy," Abdel-Samee said. "May god give her the strength to overcome the passing of her daughter but this is another issue. Parents won't acknowledge or admit this is happening. But it did happen."
"There is still a long way to go to eliminate this harmful practice that violates the rights of women and girls," the U.N. said in a statement.
"There is no moral, religious or health reason to cut or mutilate any girl or woman."
FGM has no medical benefits, according to the U.N. and can cause lifelong physical and emotional trauma for the millions of women forced each year to undergo the procedure. But disseminating a clear message against the practice has proven a challenge for the Egyptian government.
The National Council of Women, a state body, said in a statement: "All state institutions must be involved in erasing this hideous practice from the community's memory. The role of all institutions including religious institutions in raising awareness is important."
Efforts to end FGM have resulted in some progress. The percentage of girls aged 15 to 17 who have had the procedure has dropped from 74.4% in 2008 to 61% in 2014, according to the Egyptian Ministry of Health.
In January of last year a doctor was sentenced on charges related to mutilating a girl -- the first conviction of its kind since the 2008 ban went into effect. But rights campaigners tell CNN that enforcement is difficult and does not address the often collective acceptance of FGM in some communities.
"Criminalization isn't enough," said Dalia Abdelhameed, the gender and women's rights officer at the Egypt-based Egyptian Initiative for Personal Rights. "The national campaigns have to confront the root causes that propel the families to circumcise their daughters in hopes of controlling their sexual desires."