The Lebanese army detained over the weekend a main suspect in the recent killing of an Irish UN peacekeeper in a move coordinated with the Hezbollah party, two security sources and a Hezbollah spokesperson said.

The man is a supporter of the Iran-backed party, but not a member of the group, the Hezbollah spokesperson told Reuters.

The security sources said the man was suspected of firing shots at a United Nations Interim Force in Lebanon (UNIFIL) vehicle that was travelling through south Lebanon on Dec. 15.

Private Sean Rooney, 23, was killed in the incident, the first fatal attack on UN peacekeepers in Lebanon since 2015.

Hezbollah has officially denied involvement in the incident, calling the killing an “unintentional incident” that took place solely between the town’s residents and UNIFIL.

On Dec. 16, Ireland’s then-foreign and defense minister Simon Coveney told state broadcaster RTE that he did not accept Hezbollah’s assurances that it had no involvement.

“We don’t accept any assurances until we have a full investigation concluded to establish the full truth,” he said.

UNIFIL has operated in Lebanon since 1978 to maintain peace along its border with Israel. It was expanded after a UN resolution that halted the 2006 Israel-Hezbollah war in Lebanon.