The porta potty exploded after someone lit a bundle of fireworks and tossed them inside.
Several people at a skateboard park in University Place saw the portable bathroom explode June 29, then two men hurriedly drive away. Another two men walked off.
Witnesses recalled the four standing together just before an explosion rang out and debris from the porta potty sprayed the park grounds.
Police pulled over a car as it left the skate park. Two 18-year-old men and an underage boy were inside. Mortar shells, sparklers and duct tape were found inside the car as well.
On Thursday, Pierce County prosecutors charged the two men with second-degree malicious mischief and third-degree malicious explosion of a substance. Both pleaded not guilty and were ordered jailed in lieu of $10,000 bail.
The man accused of throwing the fireworks inside the porta potty told sheriff's deputies he was hanging out at the skate park but knew nothing about an explosion, according to charging papers.
His friend said they duct-taped 96 sparklers together and threw them inside the porta potty.
The juvenile boy told deputies the trio was going to another location to light off the rest of the fireworks, records show.