The dismembered remains of a Syrian girl were discovered by chance in a local morgue last week by her mother who was expecting to identify the dead girl's activist brother
Zainab Al Hosni, 18, from the city of Homs had been decapitated, her arms cut off, and had her skin removed, Amnesty International said.
She was abducted by men believed to belong to loyalist security forces in July in an apparent attempt to pressure her activist brother, Mohammad Deeb Al Hosni, to turn himself in, Amnesty said.
Both were killed, bringing Amnesty's number of reported custodial deaths to 103 since mass protests against the regime of President Bashar al-Assad began in March.
"If it is confirmed that Zainab was in custody when she died, this would be one of the most disturbing cases of a death in detention we have seen so far," Philip Luther, Amnesty's deputy director for the Middle East and North Africa, said in a statement.
Zainab is believed to be the first woman killed by security forces while in custody.
Her brother Mohammad, 27, had been organising protests in the cental city of Homs, which has been a focal point for opposition demonstrations calling for political reform.
His body showed signs of torture including bruising on the back, and cigarette burns on the body. He had been shot in the right arm, right leg, and three times in the chest, Amnesty said.
His mother unexpectedly discovered Zainab's mutilated body at the same hospital. The goup said she was made to sign a document saying her children had been kidnapped and killed by 'an armed gang.'
Meanwhile, forces loyal to Assad killed at least 13 civilians in Homs over the weekend.
"Twelve civilians were killed in Qusseir during search operations for people wanted by the army and security forces in this locality," the Syrian Observatory for Human Rights told AFP in Nicosia.
Security forces shot and killed another person in the nearby city of Hama, it added.
In addition, the bodies of two people who had gone missing in recent days were handed over to their families by Syrian authorities, while another died of wounds suffered on Friday.
The weekend's deaths came on the heels of nine others killed by live fire during anti-regime protests Friday afternoon, the London-based observatory said.
More than 2,700 anti-regime protesters and activists have been killed in a brutal crackdown since March 15.