NASHVILLE, Tenn. (Reuters) - A 3-year-old boy shot his 18-month-old brother in the head in rural Tennessee and investigators are trying to determine how he gained access to the handgun, authorities said.

Coffee County Sheriff Steve Graves said the younger boy was struck by a bullet fired from a .25-caliber pistol on Tuesday and was awake and alert afterward.

The child was hospitalized at Vanderbilt University Medical Center in Nashville after the shooting in Coffee County, Tennessee, which is about 75 miles southeast of Nashville. The hospital has not released his condition.

"We've still got interviews to determine how the child came about getting the gun," said Graves, who declined to identify the children. "There's no doubt the 3-year-old shouldn't have had access to it."

(Reporting by Tim Ghianni in Nashville, Tennessee; Editing by David Bailey and Mohammad Zargham)