But do it soon. And investigate the nomads too, while you're at it. Sounds rather tedious, but here are a few ideas how to build an adventure around it. The heroes are assigned to follow-up. But the problems start even before they leave.
Several mishaps occur and a whole lot of weird incidents start to happen.
The agents meet a strange man at the airport, warning them not to go. The hotel the agents stay at the night before heading into the jungle, they find a knife in stuck in the door when they go up to the rooms. At work here is a cult of Mayan warriors whose ancestors fled Mexico when the empire fell. They protect the secret of their city and cult from outsiders, willing to kill to do so. To go even further, the city wasn't even abandoned.