The interior ministry said the girl, named as Spozhmai, was detained before she detonated the vest near a police checkpoint in the district of Khanashin, but some local officials said the trigger button did not work when she pressed it.
Speaking yesterday at a press conference, the girl said she had been ordered by her brother to undertake the suicide mission but had decided at the last minute not to go through with it.
"I went past a river and decided to drop the vest. My brother fled and police arrested me. He had told me nothing would happen to me."
During the press conference with local media in the Helmand provincial capital of Lashkar Gah, the girl -- wearing a blue dress and red and grey headscarf -- spoke briefly about the incident on Sunday night.
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"We have appointed a team to investigate this," Omar Zwak, the Helmand governor's spokesman, told AFP.
Zwak said the girl gave police varying versions of the events, at one stage saying that she had crossed the river alone and got cold and wet before being arrested and then secretly returning home to put on dry clothes.
She did not explain why the police allowed her home.
The girl, who is now in police custody, also told police she did not want to live at home any longer because her brother and father were Taliban fighters and would kill her, and that she also did not get on with her stepmother.
The interior ministry's account said her brother was a Taliban commander who coerced her into putting on the vest and walking towards the police post.
Female child suicide bombers are extremely rare in Afghanistan, but boys have been used to evade security and detonate explosives -- a tactic also seen in Iraq where disabled children were sometimes persuaded to carry suicide vests.