(CNN) -- U.S. Sen. John McCain is blaming illegal immigrants for starting some of the wildfires that have scorched hundreds of thousands of acres in Arizona.

"There is substantial evidence that some of these fires have been caused by people who have crossed our border illegally," McCain, R-Arizona, said Saturday at a press conference. "The answer to that part of the problem is to get a secure border."

The Arizona senator, however, did not say what the evidence is, prompting a swift rebuke from Latino civil rights advocates.

"It's easier to fan the flames of intolerance, especially in Arizona," said Randy Parraz, a civil rights advocate who ran unsuccessfully against McCain as a Democratic candidate in 2010.

Parraz called McCain's remarks "careless and reckless" but not entirely surprising given the political climate in Arizona. The Latino advocate is co-founder of Citizens for a Better Arizona, a group trying the recall the legislator who authored the state's controversial anti-illegal immigration law.

Parraz said McCain "should know better" than to make such an accusation without presenting any facts.

McCain said that illegal immigrants set such fires either to send signals, keep warm or distract law enforcement agents. But he did not specify which fires allegedly had been started by illegal immigrants, nor did he identify his sources or provide details of the "substantial" evidence he cited.

Firefighters are currently battling five wildfires that have burned a combined 732,427 acres in Arizona, according to InciWeb, an online interagency database that tracks fires, floods and other disasters. The fires are under investigation and suspects have not been named. However, local media outlets have reported anecdotal cases of fires breaking out in areas where illegal immigrants have been known to cross the border.

Parraz said it is particularly distressing that immigrants are being blamed for destructive fires at a time when many are also being targeted given the state's unemployment, foreclosure and other economic issues.

"People are looking for someone to blame," he said, claiming it is too easy and convenient to target what he called one of Arizona's "most vulnerable populations."

Angelo Falcon, the president of the National Institute for Latino Policy, criticized McCain for what he called "increasingly blatant" political opportunism.

"The degree of irresponsible political pandering by Sen. McCain has no limits," Falcon said in an email to CNN. "With the lack of evidence, he might as well also blame aliens from outer space for the fires."