Marcelo Pecci, 45, was felled by two shots while with his wife on the shores of the Caribbean island Baru on May 10.

Pecci had specialised in tackling organised crime, drug trafficking, money laundering and terror financing.

His wife, Paraguayan journalist Claudia Aguilera, was not injured in the attack. The couple was expecting a child.

The killing was carried out by two men who arrived on a jet ski. Authorities reconstructed the crime through security camera footage.

One of the four men who confessed is Venezuelan, while the others are Colombian, prosecutors said in a statement.

Colombian authorities said they have not identified the mastermind of the killing, but that the hit squad was paid US$530,000.

Pecci investigated crime gangs based in Brazil and money launderers in an area where the borders of Paraguay, Brazil and Argentina converge.