Medford, Ore. -- Medford Police said they're treating Monday afternoon's fire in downtown as an arson, bringing the number of arson cases to 18 since late June.
Monday's fire damaged some siding to a house on Fifth and Ivy and burned some grass and a sewage pipe. While the cause is technically undetermined, police are treating it as arson.
Authorities believe all 18 fires set since late June are related and set by the same person.
Police said the arsonist is setting fire to random locations with nothing to gain, making it harder for them to track down the person responsible.
"It's not like they're doing this to get insurance money or to get back at somebody," began Lieutenant Mike Budreau with the Medford Police Department.
"In those cases we could form a nexus between the victim and the suspect and that's how we solve a lot of our crime is a motive, finding a motive," he continued.
So what is it that motivates a serial arsonist? Why do they do it? NBC 5 News spoke with psychologists to figure out what fuels fire-setters.
Clinical Psychologist Dr. Karen Muller has worked with arsonists in her career. She said while every arsonist is different, there's one seemingly universal characteristic.
"At least with the ones that I knew and with most of the data, anger is the most prevalent theme and you most often get anger in young offenders from abuse," said Muller.
According to Muller, arsonists are also more likely to be young men. Each arsonist may have different reasons for setting fires.
"There are some that have sexual gratification association. I actually never met one that did have that. There is the fascination with fire aspect of it," Muller said.
Forensic Psychologist Dr. Eric Morrell also said addiction could play a role in certain cases, especially in the case of a serial arsonist. Muller agrees.
"They do seem to keep doing it until they're caught," said Muller.
Morrell also said some arsonists could have a tendency toward anti-social, aggressive behavior toward people or animals. In addition, a serial arsonist could get a rush from repeatedly getting away with their behavior.
Back in March, Medford Police arrested Manoah Hall, the man suspected of starting a string of vacant home fires spanning back to 2011. According to police, Hall's YouTube page had uploaded videos of firetrucks responding to fires, some of them police believe Hall started himself.
"Sometimes it's a fascination with firetrucks, sometimes it's a fascination with fire, sometimes it's a fascination with the chaos that ensues," said Budreau.
According to Morrell, an arsonist could have Pyromania, a condition where someone is driven to start fires in order to relieve tension
Relief is something Muller said is common in some arsonists she's spoken to during the course of her career.
"It was more about the satisfaction of the fire or the release of the fire that outlet for their anger," she said.
However in the end, Muller and Budreau agree not much is known about arsonists because they're rarely caught.
"This is not a crime you're going to brag about or talk about to others," said Budreau.
The fact that arsonists typically evade arrest, is a trend police hope will not hold true for the serial arsonist targeting their town. Budreau said with each additional arson, there will be another opportunity to catch the person responsible.
So far, police said they believe commonalities in most of the 18 arson investigations include a consistent geographical area centered in West Medford and the Downtown area, as well as locations with alley access.
Police are asking the public to call 911 and report any suspicious activity near or around alleyways in Medford.
Copyright KOBI-TV. All rights reserved unless otherwise stated. This material may not be published, broadcast, rewritten, or redistributed without written permission.