The unnamed woman was unaware that the device posed an immediate threat to her life, so she drove it to a nearby fire station herself.
There, firefighters contacted the Tempe Police bomb squad, which sent over investigators to examine the device.
Pooley said the device was "ready to go" and just needed a trigger.
Prater was subsequently arrested on charges of misconduct involving weapons and possessing a prohibited weapon.
The purpose of the device remains unclear.
Pooley commended the cleaning lady on her quick action, saying that although she risked her life to deliver the device to the authorities, she ultimately "did the right thing."