The Fish and Game biologists were identified as Larry Barrett, 47, and Danielle Schiff, 34. The name of the pilot was not immediately released.
The Department of Fish and Game had chartered the aircraft from Clarkston, Wash.-based Leading Edge Aviation to use on a trip counting salmon spawning nests on the Selway River.
Officials said the aircraft appeared to have experienced some sort of mechanical failure, causing it to crash just a block from Main Street in the small north-central Idaho town of Kamiah.
No one on ground was injured.
CBS affiliate KBOI in Boise reports that eyewitnesses say the helicopter was making popping sounds and that material was coming off it before it crashed. Those who saw it come down say it crashed just three blocks from a school, and that the pilot struggled to keep it from crashing into that school or other residences.
Witness Jim Emmert said he was at home when he heard loud rhythmic banging, "like a rock-crusher, but with metal parts."
He ran to his deck of his home and realized the noise was coming from the helicopter heading east through Clearwater Valley.
"The thing was sounding like there was some serious metal destruction going on," Emmert said. "It was maybe 800 feet off the valley floor, and then it kind of hesitated in its forward momentum before sliding backward."
The helicopter lost a few hundred feet of altitude and flipped upside down, with its rotors toward the ground, before the pilot briefly managed to get it upright and level again.
"It started sliding to the left and downward and going down too rapidly," Emmert said. "He was headed toward some power lines, and I perceived that he banked it toward the right to avoid them."
The helicopter then collided with the travel trailer on the ground.
"I saw white parts coming off of it," Emmert said. "It made a crumpled sound - like paper being crumpled - and then a loud crash and an explosion. And then nothing."
Kamiah resident Mick Landmark said the helicopter looked like it clipped the eaves of a U.S. Forest Service-owned house before colliding with the trailer about a block from his downtown insurance office.
The Lewis County sheriff's office was investigating the crash in north-central Idaho.