An Irish peacekeeper deployed along the Lebanese border with Israel was shot and killed, and three others were injured. The attack is being investigated.

A U.N. peacekeeper in southern Lebanon was shot and killed Wednesday night when the armored vehicle he was in came under gunfire in an incident that is under investigation.

The peacekeepers, part of a United Nations mission deployed along the Lebanese border with Israel, were not on patrol at the time of the episode, which occurred outside their area of operations in the south.

Pvt. Seán Rooney, a member of the Irish battalion, was pronounced dead on arrival at the hospital.

Seven other peacekeepers were traveling with him from southern Lebanon to Beirut. Another peacekeeper is in serious condition and two others received minor injuries.

The United Nations Interim Force in Lebanon, or Unifil, has been deployed in Lebanon since 1978. Its mandate was expanded to patrolling the border after the 2006 war between Hezbollah, the Lebanese militant group, and Israel.

“At the moment, details are sparse and conflicting,” the U.N. force said in a statement. It is investigating the killing in coordination with the Lebanese Army.

More than 10,000 U.N. peacekeepers from 48 countries patrol and help keep the peace along the 75-mile frontier between Lebanon and Israel, known as the Blue Line.

The Blue Line is a demarcation set by the U.N. to confirm Israel’s withdrawal from Lebanon in 2000, ending its 15-year occupation of southern Lebanon.

The border between the two countries, who remain in a declared war with each other, remains unresolved and in dispute, making the area a tense, and at times dangerous, place with outbreaks of conflict occasionally flaring.

The area is lined with minefields and barbed-wire fencing and surveilled by drones with the Lebanese Army and Hezbollah on one side and Israel on the other.

“We are in a peacekeeping mission and a peacekeeping mission is by nature in an area surrounded by a volatile situation and instability,” said Andrea Tenenti, the Unifil spokesman. “But in the last several months the situation in south Lebanon has been calm, and we haven’t suffered any incidents.”

Private Rooney’s killing is the first since 2015 when a Spanish peacekeeper was killed by Israeli fire during clashes between Israel and Hezbollah.

Since 1978, 335 Unifil peacekeepers have been killed or died while deployed in Lebanon, including of natural causes.

Southern Lebanon is a Hezbollah stronghold, but a group spokesman said they had nothing to do with the incident.