Her two companions, however, completed their grisly mission earlier this week and walked into a crowd of hundreds at Dikwa refugee camp in northeast Nigeria and blew themselves up, killing 58 people.
Later found by local self-defence forces, the girl's tearful account is one of the first indications that at least some of the child bombers used by Boko Haram are aware that they are about to die and kill others.
"She said she was scared because she knew she would kill people. But she was also frightened of going against the instructions of the men who brought her to the camp," said Modu Awami, a self-defence fighter who helped question the girl.
She was among thousands held captive for months by the extremists, according to Algoni Lawan, a spokesman for the Ngala local government area that has many residents at the camp and who is privy to information about her interrogation by security forces.
"She confessed to our security operatives that she was worried if she went ahead and carried out the attack that she might kill her own father, who she knew was in the camp," he told The Associated Press on Thursday.
The girl tried to persuade her companions to abandon the mission, he said, "but she said she could not convince the two others to change their minds."
Her story was corroborated when she led soldiers to the unexploded vest, Awami said Thursday, speaking by phone from the refugee camp, which holds 50,000 people who have fled Boko Haram's Islamic uprising.
Captives turned into weapons
The girl is in custody and has given officials information about other planned bombings that has helped them increase security at the camp, said Satomi Ahmed, chairman of the Borno State Emergency Management Agency.
The United States on Thursday strongly condemned the bombings. State Department spokesman Mark Toner said the U.S. remains committed to assisting those afflicted by the conflict and supports efforts to provide greater protection for civilians and the regional fight against terrorism.
Boko Haram's six-year-old Islamic insurgency has killed 20,000 people, made 2.5 million homeless and spread across Nigeria's borders.
The extremists have kidnapped thousands of people and the increasing number of suicide bombings by girls and children have raised fears they are turning some captives into weapons. An army bomb disposal expert has told the AP that some suicide bombs are detonated remotely, so the carriers may not have control over when the bomb goes off.
The latest atrocity blamed on Boko Haram extremists was committed against people who had been driven from their homes by the insurgents and had spent a year across the border in Cameroon.
Some 12,000 of them had only returned to Nigeria in January when soldiers declared the area safe. The scene of the killings is 50 kilometres from the border with Cameroon and 85 kilometres northeast of Maiduguri, the biggest city in the northeast and birthplace of Boko Haram.
Such attacks make it difficult for the government to persuade people to return home. The extremists have also razed homes and businesses, destroyed wells and boreholes and stolen livestock and seed grains that farmers need to start their life again.