Indonesian authorities are investigating deadly attacks by insurgents in Papua said to have claimed the lives of 19 workers and one soldier.

Colonel Muhammad Aidi, a spokesperson for the Indonesian military, said members of an armed separatist group were commemorating their independence day in Nduga district in the restive province in the country’s east when a construction worker reportedly took a photo of them, angering the group and triggering the attack, the BBC reported.

A total of 19 workers plus one soldier are believed to have been killed on Sunday and Monday, which would make the combined attacks among the most deadly in the history of Papua.

According to an early report by Straits Times, 31 construction workers were killed in the attack. But the toll was later revised down by officials.

Aidi said the bodies of the workers were found near a bridge the workers were building.

On Monday, police and soldiers got to the area to investigate the first attack but were met with gunfire from the separatists, who oppose Indonesian control over their traditional lands and development in the province, which has been encouraged by President Widodo.

One soldier was killed in this encounter, while another was wounded.

Papua declared itself independent from Dutch rule in 1961. However, Indonesia occupied the region in 1963 and six years later its takeover was endorsed by the United Nations.

The Indonesian government has imposed a brutal and a tight hold on Papua, which has abundant natural resources.

Foreign journalists are heavily restricted from visiting the province.