A Palestinian judicial official says a special investigator who tracked Palestinians who sold land to Israelis has died in custody under mysterious circumstances.

The official said 49-year-old Osama Mansour fell on Sunday from the third floor of the Palestinian military intelligence building in the West Bank city of Ramallah, where he was being held. The cause of the fatal fall was unclear.

The official said Mansour was working in the attorney-general's office and was arrested 25 days ago, reportedly on suspicion of conducting land transactions with Israelis.

According to a report in the French news agency AFP, Mansour's brother said he believed his brother had been murdered.

"The family visited him on Friday and he was in good psychological condition with no indication that he was in any sort of state to commit suicide, so we demand an autopsy to find out who killed my brother," Mansour's brother told AFP.

Palestinians convicted of selling land to Israelis face the death penalty. Palestinian society views such acts as treason because it weakens their quest for an independent state in the West Bank and other lands Israel occupied in 1967.