15 people were killed in a prison in Amazonas, northern Brazil, on Sunday, after clashes broke out among prison inmates, according to the provincial prison authority.

The clashes broke out in the morning during the visit, in the prison about 28 km from Manaus, the state capital.

“It was a battle between prisoners,” Colonel Marcos Venecius Almeida told a news conference, adding that an investigation had been opened to determine the cause of the fighting.

Almeida confirmed that the authorities responded within minutes to the violence, which prevented the worst.

In January 2017, the prison itself was the scene of a 20-hour rebellion that left 56 people dead.