Alonzo Crim Open Campus High School was locked down just before 2 p.m. Tuesday after a device was found.
It is not clear what time the lockdown ended, but students have been dismissed.
Atlanta Public Schools spokeswoman Kimberly Willis Green said a bottle was set on fire and thrown into the parking lot at the school.
Atlanta police initially reported the device was a Molotov cocktail and that SWAT and Homeland Security were notified.
Channel 2 Action News reported the device was a water bottle with a rag sticking out of the bottle’s opening. The rag was on fire and a janitor extinguished the flame.
Green said the incident remains under investigation.
