A quad bike on fire caused a blaze that spread across more than 500 hectares (1,235 acres) of protected moorland near Huddersfield, police said.
Up to 50 firefighters spent 24 hours tackling the flames at Marsden Moor after the fire started at about 1200 BST on Saturday.
National Trust managers fear rare birds, hares and vegetation could have perished due to the speed it spread.
Police said they were looking for the owners of the quad bike.
Officers said their investigations were at an early stage and it was not yet known whether the fire was started deliberately.
Firefighters said nine engines were sent to tackle two separate fires on the moor, which is designated a Site of Specific Scientific Interest.
The fire is believed to have spread quickly due to dry and windy conditions.
National Trust countryside manager Gemma Wren said: "We believe birds and hares will have been wiped out, we saw lots of them trying to get out of the way."
The National Trust scrambled a helicopter with a water bucket to help douse the flames.