The revelation marks the first known instance of Palestinian activists being targeted by the military-grade Pegasus spyware. Its use against journalists, rights activists and political dissidents from Mexico to Saudi Arabia has been documented since 2015.

A successful Pegasus infection surreptitiously gives intruders access to everything a person stores and does on their phone, including real-time communications.

It's not clear who placed the NSO spyware on the activists' phones, said the researcher who first detected it, Mohammed al-Maskati of the nonprofit Frontline Defenders.

Shortly after the first two intrusions were identified in mid-October, Israeli Defense Minister Benny Gantz declared six Palestinian civil society groups to be terrorist organizations. Ireland-based Frontline Defenders and at least two of the victims say they consider Israel the main suspect and believe the designation may have been timed to try to overshadow the hacks' discovery, though they have provided no evidence to substantiate those assertions.

Israel has provided little evidence publicly to support the terrorism designation, which the Palestinian groups say aims to dry up their funding and muzzle opposition to Israeli military rule. Three of the hacked Palestinians work for the civil society groups. The others do not, and wish to remain anonymous, Frontline Defenders says.

The forensic findings, independently confirmed by security researchers from Amnesty International and the University of Toronto's Citizen Lab in a joint technical report, come as NSO Group faces growing condemnation over the abuse of its spyware and Israel takes heat for lax oversight of its digital surveillance industry.

Last week, the Biden administration blacklisted the NSO Group and a lesser-known Israeli competitor, Candiru, barring them from U.S. technology.