The victims of a massacre in the Democratic Republic of Congo included several pregnant women, had been shot, stabbed or burned inside their homes.
A Reuters cameraman on the scene counted 37 bodies, some of them lying inside a village church.
"It is Congolese who have carried out these attacks. It was about a dispute over cows," South Kivu governor Marcellin Cishambo told Reuters.
"The problem is that everyone in this area carries a weapon."
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EnQuest said the evacuation was taking place as a precautionary measure.
Opioid use has been linked to the deaths of more than 400,000 people over the past two decades in America.
The 21-year-old man was arrested on Monday night on suspicion of two counts of murder after the incident in Milton Keynes.