(CNN)-- An explosion killed at least 21 militants in Iraq Monday, apparently when a lesson on car bombing went wrong, police said.
Police said they found a camera inside a house where the blast took place, leading them to believe someone was filming the installation of explosives into a vehicle for propaganda purposes.
After a loud explosion, security officials rushed to the scene and found body parts, mostly of young men, scattered about. Six people were critically injured. Security forces arrested four men fleeing.
Police said they believe the house was a small training base for the al Qaeda-affiliated Islamic State of Iraq and Syria (ISIS). They also found documents and flags usually used by al Qaeda-linked groups such as ISIS.
The explosion happened inside a remote house northeast of Samarra, a predominantly Sunni town about 110 kilometers (about 68 miles) north of Baghdad.
Separately, shootings and explosions across the country killed at least 14 people and wounded 29 others, police officials in Baghdad, Mosul and Ramadi told CNN.