The kidnapped man, Jafaal Sawalma, was interrogated by his captors, who accused him of transferring information to Israel used to assassinate the group's Nablus commander, Muhammad Shatawi, last week.
Sawalma was already arrested on Saturday by PA security forces and released after being questioned for two days. But Al-Aqsa Brigades members were not satisfied with the official investigation and kidnapped Sawalma.
Earlier, a senior member of the organization said that if Sawalma is found innocent, he will be released shortly, and if is found to have provided information used in Shatawi's assassination, the group will kill the hostage.
The source added that Sawalma's work for the United Nations was irrelevant.
A month ago, a German citizen who was a volunteer English teacher was kidnapped in Nablus by anonymous gunmen, and released shortly afterwards, as an act of protest against cartoons of Islam's prophet Muhammad.