RAPID CITY, S.D. — U.S. Sen. John Thune is requesting federal disaster aid for ranchers affected by a large prairie fire in western South Dakota.

Thune wrote to U.S. Agriculture Secretary Tom Vilsack on Tuesday, asking for immediate aid to ranchers affected by the Cottonwood Fire. Thune notes that many of the ranchers were hit by a winter storm three years ago.

The fire started Sunday afternoon east of Wall and has burned 48 square miles. Authorities say the blaze was human-caused.

Numerous rural buildings were burned, though no homes. Smoke from the fire cut visibility on Interstate 90 and contributed to at least two crashes. No one was hurt.

About 300 firefighters from 10 western counties fought the blaze at its peak.

This story has been corrected to remove an erroneous reference to the state fire marshal’s office investigating the fire’s cause.