Israel used U.S.-supplied white phosphorus munitions in an October attack in southern Lebanon that injured at least nine civilians in what a rights group says should be investigated as a war crime, according to a Washington Post analysis of shell fragments found in a small village.

The Pentagon requires partner militaries to acknowledge obligations under international law when they accept U.S. weapons, “including that these munitions are only to be used for lawful purposes such as signaling and smoke screening,” a U.S. defense official said, speaking on the condition of anonymity because of the sensitivity of the issue.

It is unclear when the United States delivered the munitions to Israel. The official said no white phosphorous munitions have been provided since the Oct. 7 Hamas attack.

White phosphorus fell onto several homes and ignited fires, incinerating furniture and stripping appliances to scorched metal. Remnants of the sticky, black chemical littered the ground 40 days after the attack and combusted when residents kicked at it.

In 2013, the Israeli military pledged to stop using white phosphorus on the battlefield, saying it would transition to gas-based smoke shells.

Lebanese Prime Minister Najib Mikati said on Dec. 2 that Israel’s use of the munition has “killed civilians and produced irreversible damage to more than 5 million square meters of forests and farmland, in addition to damaging thousands of olive trees.”