Humans are the primary cause of fires and start roughly six times more blazes than lightning strikes in a given year, according to the National Interagency Fire Center in Boise, Idaho, which tracked fire causes between 2001 and 2013.
Human-caused fires burn more than 2. 5 million acres each year, or nearly 4,000 square miles — an area bigger than Delaware.
Some are set intentionally by arsonists, but the rest are accidents that start with everything from sparking cars to abandoned campfires to discarded cigarette butts.
Here are some examples of some recent fires that made news and the things that started them.
Nineteen firefighters were killed last June while fighting a blaze in Arizona that was sparked by a lightning strike.
A fast-moving wildfire in California's Riverside County in 2006 killed five firefighters and burned 60 homes. Investigators determined that a serial arsonist had ignited the flames after lighting multiple, smaller fires over a period of months as he refined his technique. Raymond Lee Oyler was sentenced to death in the case.
A wildfire that erupted in Camarillo, California, last year charred 44 square miles and damaged 15 homes and several recreational campers. Fire investigators later determined that some type of roadside spark had started the blaze. The exact source was never determined. Another common cause of vehicle-related fires is dragging chains. In October 2012, a chain and a jack hanging from the back of a vehicle in Idaho caused sparks that started a 10-acre blaze.
A fire southwest of Denver in 2002 burned 215 square miles and scorched more than 130 homes. The fire started when a former U.S. Forest Service employee, Terry Barton, burned a letter in a campground. Barton served six years in prison after pleading guilty to starting the blaze. Another fast-moving wildfire in Yosemite National Park and the Sierra Foothills burned 400 square acres, destroyed dozens of homes and caused an estimated $54 million in damages. Authorities later determined that the fire was sparked by an illegal campfire set by a hunter who was camping on U.S. Forest Service land.
WELDING, MOWING AND OTHER EQUIPMENT USE
A massive fire on Catalina Island, off the Southern California coast, sent hundreds of residents fleeing the island on ferry boats in 2007 as flames spread across thousands of acres and destroyed one home. Investigators determined that a subcontractor sparked the blaze while using an open-flame torch to cut wires on the island's radio tower. He was sentenced to five years of probation and $4 million in restitution in 2009.
A 2008 blaze in Yosemite that destroyed 130 homes and caused $37 million in damage was started by a man who was shooting assault weapons in the forest. A bullet struck a rock and created sparks that started the fire. The man was sentenced to probation, community service and fines.
In 2012, an Arizona man fired a flame-throwing shotgun round into an empty soda box during target practice at a bachelor party, sparking a more than 28-square-mile wildfire. Steven Craig Shiflet received two years of probation, but did not have to pay the $4.4 million in firefighting costs associated with the blaze.
A 2012 fire in Creek County, Oklahoma, began when a man discarded a cigarette butt in dry grass in front of his house. The blaze charred more than 90 square miles and destroyed nearly 400 homes as it burned for six days. The man was charged with felony arson and unlawful burning.
In 2002, a woman who got lost on an American Indian reservation in Arizona set a fire to get the attention of a television news helicopter, causing a wildfire that merged with a second blaze and become one of the worst in Arizona history. The fire burned nearly 733 square miles of private, Forest Service and Fort Apache Reservation land in eastern Arizona before it was contained. She was not prosecuted.
In 2006, a lost hiker near Yucapia, California, set a signal fire to try to attract attention and started a fire. The blaze was extinguished quickly at 125 acres by more than 400 firefighters.
Associated Press reporters John Antczak in Los Angeles, Thomas Peipert in Denver and Josh Hoffner in Phoenix contributed to this report.