After assassinating a top Hamas commander, Muhammed Sinwar, in May 2025, the Israeli military sent a special unit into an underground complex he had used. There, they found a computer unconnected to a network — and much harder to access by Israeli operations spying on Hamas communications.

The computer held an image of a six-page memo, handwritten in Arabic, that the Israeli intelligence community believes was by his brother Yahya Sinwar, who as the powerful leader of Hamas in Gaza helped plot the Oct. 7, 2023, attack on Israel. Dated Aug. 24, 2022, it appears to be a directive from Mr. Sinwar with instructions for the assault, according to seven Israeli officials.

The memo, a copy of which was obtained by The New York Times, calls for fighters to target soldiers and civilian communities — as well as to broadcast the violent acts so as to evoke fear in Israelis and destabilize the country. Commanders then issued similar instructions on Oct. 7, according to hours of previously unreported communications between commanders and subordinates intercepted by Israel during the assault and shared with The Times.

The Israeli officials say the memo shows that Mr. Sinwar wanted his fighters to target civilians from the outset, contradicting what the group’s leadership has publicly claimed.

Although the memo does not explicitly mention plans to kidnap or kill civilians, it lays out orders for fighters to enter residential neighborhoods and set them on fire “with gasoline or diesel from a tanker.”