A five-year-old boy who fatally shot his baby brother in the US was high on cocaine while the toddler had marijuana in his system, according to prosecutors.

Sixteen-month-old Isiah Johnson died from a gunshot wound to his head on March 28 in Lafayette about 60 miles north of Indianapolis, after his brother found a handgun in the family's apartment, prosecutors said.

They added that Isiah had marijuana in his blood and his brother tested positive for cocaine.

After the shooting, police found 93 fentanyl pills, marijuana, and paraphernalia inside the apartment, they added.

Prosecutors have filed neglect and drug charges against parents Deonta Jermaine Johnson, 27, and Shatia Tiara Welch, 24, of Lafayette.

The charges were unsealed on Tuesday following their arrests the day before in LaPorte, news outlets reported.

Johnson was asleep inside the apartment at the time of the shooting, authorities have said while Welch was not present.

Welch told police she owns the gun, but she usually kept it locked in a box under her bed.

Johnson also faces a charge of obstruction of justice for removing marijuana from the apartment before police arrived after the shooting, prosecutors said.