A bomb squad responded to Brownell Elementary School in Lincoln, Nebraska, around 1:30 p.m. after a staff member found the small black device, The Lincoln Journal Star reported.
Lincoln Fire Investigator Ken Hilger told the newspaper the wick of the improvised explosive device appeared to have been previously lit but it burned out before the device could explode.
Investigators said the explosive was made from a CO2 canister.
"If it had exploded, it would have caused shrapnel to fly in all directions," reported KLIN, citing Lincoln Chief Fire Investigator Bill Moody.
Authorities seized the device and ensured it was inert, according to KLIN. Investigators planned to store the device, in case similar ones are found, and ultimately destroy it, according to The Lincoln Journal Star.
Authorities did not identify any potential suspects or motives in the case.