Apple Inc. iPhones of at least nine U.S. State Department employees were hacked by an unknown assailant using sophisticated spyware developed by the Israel-based NSO Group, according to four people familiar with the matter.

The hacks, which took place in the last several months, hit U.S. officials either based in Uganda or focused on matters concerning the East African country, two of the sources said.

The intrusions, first reported here, represent the widest known hacks of U.S. officials through NSO technology. Previously, a list of numbers with potential targets including some American officials surfaced in reporting on NSO, but it was not clear whether intrusions were always tried or succeeded.