By Sam Wilson
BBC News website
As Portugal and Spain survey the cost of devastating forest fires, there is dismay that so much destruction has apparently been caused deliberately.
Arsonists may be motivated by excitement, revenge, or profit
Both countries have arrested more than 100 people on suspicion of arson this summer.
Many have been teenaged boys but some are firefighters themselves.
More than 30 people have been killed by the blazes, while great swathes of countryside and scores of homes have been consumed.
While lightning and other environmental factors are responsible for some fires, the majority are started by humans.
Many are down to careless accidents - a discarded cigarette or neglected campfire - but a large number, according to experts, are started in cold blood.
"Generally about 30% of human-caused wildfires are deliberately set," says Timothy Huff, a former FBI profiler specialising in arson, who has investigated hundreds of fires in California and elsewhere.
The figures vary between areas, but Spain's Civil Guard believes at least 125 of the 829 fires it has investigated this summer were started deliberately.
Investigators say developers may start fires to clear land
The 33-year-old head of a Spanish fire fighting unit has been accused of setting about 15 fires in the Galicia region.
A Spanish website reported that he was put under surveillance after expressing anger at not being assigned to a local brigade.
Mr Huff lists six potential motives for arsonists: revenge, excitement, profit, vandalism, extremism, and to cover another crime.
"Excitement is one of the big ones for wildfires," he tells the BBC News website.
This covers not only teenaged boys, getting a thrill by watching a fire take hold and spread, but those motivated by the "hero factor" - people who want to be involved and to take the credit for fighting the blaze.
Also "revenge sometimes figures", says Mr Huff.
"And here there's the overriding factor of power - that is, the power to get revenge against a person that's offended them, or a group or institution, or society in general.
"These are set by persons who believe they've got a bum deal and want to get back against the world."
Crime of stealth
There may be another incentive for part-time or retained firemen to start the very thing they are supposed to stop - profit.
Miguel Angel Soto, of the environmental organisation Greenpeace, says the phenomenon is comparatively rare, but adds: "The temptation is there because firemen are paid according to the number of fires they fight."
But it is not just firefighters who can profit from fires.
It is harder to work out who started a fire than how it was started
Suspicion might also fall on firms who rent out fire-fighting equipment, hunters who want to clear brush, or property developers frustrated by environmental protection for forests.
"As long as there are people who make a living from fires, there will be an incentive to set wildfires," forest engineer Ricardo Terra Santos, who has co-authored a book on the state of Portugal's forests, told the AFP news agency.
A reader of the BBC website, Miguel Simoes, writes: "My family has several forest lands in the Portuguese Douro valley (where port wine is produced) that are burned every year because land's value is increasing each year."
In Spain, the government is considering legislation that would forbid development of burned land for at least 30 years, to prevent developers from profiting.
Investigators can often get an idea of how arson was committed, but identifying and prosecuting the culprit is much harder.
"Arson in general is very difficult to prosecute, because it's a crime of stealth," says Alan Clark, executive director of the International Association of Arson Investigators.
"With wildland fires there are usually not witnesses and they take place in remote areas. So the detection rate is not so high."
The deadly risk of starting a fire may seem obvious to some, but Mr Huff says the arsonist often does not intend to endanger life.
Elderly homeowners, local volunteers, and many firefighters have perished this year in Spain and Portugal.
"That's the rub," says Mr Huff. "When the arsonist strikes the match, he doesn't really know the ultimate consequences."