An investigation in underway after a total of 54 acres of federally protected land in Starr County were burned in two separate fires.

The fires started on Sunday night along the Rio Grande near La Grulla. La Grulla firefighter Josue Martinez said the fires were a challenge to contain.

“Due to the terrain, our vehicles were not [able] to enter," Martinez said.

Firefighters had to fight fire with fire. 

“Some areas were starting to rekindle again, so we decided to conduct a back burn,” Martinez said.

According to Martinez, firefighters intentionally lit parts of the riverbank on fire — using the wind to their advantage.

As of Tuesday, there were still several smoldering hotspots on the riverbank, and firefighters told Channel 5 News they could stay there for four more days.

Officials are still working to find out what sparked the fires, but haven't ruled out foul play.

Martinez said he hopes people remember the ground is still hot and dry everywhere else.

Starr County currently has a burn ban in place, and fireworks are not allowed.