People entering the U.S. illegally from Mexico are believed responsible for more than one-third of human-ignited wildfires in Arizona over a five-year period, according to a government report that could stoke congressional debate over illegal immigration.

Sen. John McCain (R-Ariz.) said the Government Accountability Office report supports remarks he made earlier this year after his state was hit hard by wildfires. At the time, McCain was accused of "scapegoating" immigrants.

"I hope this report is a lesson to the activists and public officials that would prefer to engage in partisan character attacks rather than help focus the discussion on the vital need to secure our southern border," he said in a statement.

Illegal immigrants are believed to have started 30 of 77 fires that were investigated from 2006 through 2010, according to the report by the GAO, the investigative arm of Congress.

Federal land management agencies, however, did not investigate all 422 human-caused fires on federal and tribal land, as called for by federal policy.

"Only 18% of fires on federal land during the five-year study period were actually investigated, and thus, the number and size of fires linked to illegal border crossers may actually be higher," McCain said.

Of the 30 fires, nine burned more than 100 acres each, 16 burned 10 to 100 acres, and five burned fewer than 10 acres, according to the report.

Efforts to signal for help, provide warmth or cook food appear to be the source of the fires, according to the report. One 2006 fire that burned about 170 acres started after an injured border crosser signaled his need for help. The causes of some of the fires are not known, but the report noted that some occurred in areas known for drug smuggling.

"The presence of illegal border crossers has complicated fire suppression activities in the Arizona border region," the report said, adding that it has "increased concern about firefighter safety, and, in some instances, has required firefighters to change or limit the tactics they use in suppressing fires."

Only a limited number of fires were studied because of the lack of investigators, according to the GAO report, which could set off a congressional debate over whether federal agencies are receiving enough money from Congress to prevent fires. The report notes that the percentage of fires caused by human activity in Arizona is "consistent with the national average."

"In a time of constrained resources and competing needs, we recognize that investigating all human-caused wildland fires in the Arizona border region may not be feasible," the report notes.

The report urges officials in Arizona to look at a program in California aimed at reducing fires from illegal immigration. Cleveland National Forest has a crew that hikes trails known to be used by illegal border crossers and extinguishes abandoned campfires, according to the report. In 2008 alone, the crew extinguished 101 abandoned campfires that the report said could have grown into larger, more damaging fires.

The GAO study began in 2010, so it didn't take into account the 2011 fire season, the worst in Arizona history, that McCain said included two fires that destroyed more than 60 homes.

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Photo: A saguaro cactus is seen in silhouette with a 2000 fire in Tucson in the background. Credit: David Sanders / Arizona Daily Star / Associated Press