Eleven Colombian military personnel were missing and six injured after a helicopter crashed in the southeast of the country during an operation against guerrillas, the army said on Tuesday.

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The Blackhawk helicopter was carrying 17 military personnel when it went down. The armed forces high command did not reveal whether it was shot down or an accident.

Authorities found the helicopter in a stretch of the river Inirida in Guaviare state, an area where dissident former Revolutionary Armed Forces of Colombia guerrillas are active.

FARC rebels laid down their arms in a historic 2016 peace deal that ended a half-century of conflict and turned the guerrillas into a communist political party.

But more than 2,000 dissidents, according to authorities, refused to join the 13,000 rebels that signed the peace deal and continue to resist the government, financing themselves through drug-trafficking and illegal mining.

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The army said the helicopter was taking part in an operation against dissidents in an area where there are drug plantations.

The army’s air assault division said in a statement it was “on-site beginning the corresponding investigations to determine the circumstances regarding the time, manner and place that the events occurred.”

The more than half-century conflict left nine million people dead, missing or displaced.