The FBI is reported to be following several so-called promising leads in the investigation into the killing of two American teachers near the Freeport mine in the Indonesian province of Papua.
The teachers died, along with an Indonesian colleague, when a convoy of cars they were travelling in was fired on.
More than half a dozen people were injured in the attack.
The Far Eastern Economic Review says six FBI agents, the largest of the four teams to have visited so far, returned to Papua in late February and are staying for a month.
The US has focussed on the case because Jakarta's failure to bring anyone to justice is putting pressure on relations between Indonesia and the US.