Three Palestinians jailed for allegedly collaborating with Israel have been killed in attacks on a prison and hospital in Gaza City.
The attacks came despite a heavy guard at the hospital
Grenades were hurled into a cell, mortally wounding one of the occupants and injuring at least five others, security sources said.
Gunmen later twice raided the hospital where the injured men were taken, killing two of them in their beds.
The violence comes amid turmoil over who runs the Palestinian police.
Palestinian sources suggested that a guard had thrown the grenades which exploded at the prison in a security compound on Monday morning, killing prisoner Musa Ouda.
The injured were taken to the emergency ward at Gaza City's main hospital, al-Shifa.
Masked gunmen later walked into the hospital on two occasions, killing Mahmud al-Sharif and Walid Hamdiya in their beds as other militants kept watch in the street outside, witnesses said.
At least two of the men were accused of aiding Israeli intelligence agents by providing information on wanted militants.
Mr Hamdiya, a former member of the Hamas militant movement, confessed in 2002 to having worked with the Israelis in the run-up to the assassination of a senior Hamas figure in the mid-1990s, Palestinian security sources said.
Mr Sharif was reportedly convicted in 1999 of assisting the Israelis in their assassination of a leading member of the Islamic Jihad movement.
Members of Mr Sharif's extended family went into the streets to announce that one of their relatives had carried out the killing.
The BBC's Alan Johnston in Gaza says the allegation that Mr Sharif gave information to Israel would have been a great source of shame to his relatives, and they may have acted to try to restore the family's honour.
The killings come amid a power struggle within the Palestinian Authority over who has control of its security forces.