Eight vultures on the Israeli-occupied Golan Heights - about half the birds' population there - have been poisoned to death, Israeli officials say.
The bodies of the griffon vultures were found on Friday morning, the Israel Nature and Parks Authority (INPA) said.
It is not known what poison killed the birds or who was behind the attack. An investigation is under way.
Officials had been trying to increase the vulture count on the rocky plateau after their numbers sharply declined.
The killing of so many of the species was a "mortal blow" to Israel's birds of prey population, INPA director Shaul Goldstein told AFP news agency.
The INPA said it was all the worse because it had happened during nesting season, meaning eggs now might not hatch and chicks might not survive.
The authority said it would do everything possible to find out who was responsible and bring them to justice.
Two more vultures found poisoned but alive are being treated at a wildlife clinic.
The Golan Heights are home to a range of wildlife, including types of birds of prey, although the vulture population of the Israeli-occupied part has dropped precipitously over the past 20 years.
Many have been poisoned, allegedly by local farmers whose herds are threatened by predators, Israeli news website Walla says.
Most of the Syrian Golan Heights has been occupied by Israel since the 1967 Middle East war. In March, the US became the first country to recognise Israeli sovereignty over the area since Israel effectively annexed it in 1981.