With the Islamic State of Iraq and Syria growing in the Middle East, nearly half of Americans think the country is less safe than it was before Sept. 11, 2001, according to a recent poll.Some Republicans have expressed concern that the southern border is so porous, members of the extremist group could slip into the United States from Mexico. And a few have said such a plot is already in the works — but federal agencies don't agree.Rep. Trent Franks, R-Ariz., said in a recent phone conference with conservative nonprofit Staying True to America's National Destiny that his state faces an imminent threat."It is true that we know that ISIS is present in Ciudad Juárez or they were within the last few weeks," Franks said. "So there's no question that they have designs on trying to come into Arizona."Several other politicians have made similar claims, including Florida Sen. Marco Rubio.With Franks and others claiming that the threat is certain and looming, we decided to see whether their evidence is credible. Based on our talks with experts and statements from law enforcement officials, the possibility of ISIS attacking the United States by crossing the southern border seems unlikely. The certainty that Franks suggests about what we "know" is wrong.• • •In a statement to PolitiFact, Franks said he got the information from a report circulating on the Internet. Franks has since learned that Homeland Security believes the report was "overstated," and his subsequent comments reflect that.The report is from Judicial Watch, a conservative media website, and it claims that "Islamic terrorist groups are operating in the Mexican border city of Ciudad Juárez and planning to attack the United States with car bombs or other vehicle borne improvised explosive devices," citing several anonymous "high-level federal law enforcement, intelligence and other sources."Without knowing anything about Judicial Watch's sources — such as rank or agency — it's hard for us to assess the article's credibility. In an interview with PolitiFact, Judicial Watch president Tom Fitton would not go into further detail.The article says the Department of Homeland Security did not respond to multiple requests.We asked Homeland Security about the threat in Ciudad Juárez, and they said, "There is no credible intelligence to suggest that there is an active plot by (ISIS) to attempt to cross the southern border."The topic came up a few times in recent congressional hearings.In a Sept. 10 hearing before the House Homeland Security Border and Maritime Security Subcommittee, Rep. Lou Barletta, R-Pa., said, "We know that terrorist networks have been using our porous southern border and a broken immigration system to enter the United States."DHS deputy undersecretary for analysis Jennifer Lasley responded."To date, we have not had credible reporting that either Hezbollah or any other terrorist group has been taking advantage of our borders to move individuals in and out," she said.Later in the same hearing, Rep. Beto O'Rourke, D-Texas, discussed his conversations with DHS, in which administrators told him that there has never been evidence of terrorists entering the United States via the southern border. He posed the same question to DHS administrators present at the hearing.In response, John Wagner, acting assistant commissioner for Custom and Border Patrol's Office of Field Operations, said it is much more likely that ISIS and other Islamic extremists would come to the United States by commercial plane. Wagner said the number of people suspected in terrorist activity caught on the southern border is in the tens, while those on commercial jets is in the thousands.Both Rep. Franks and Fitton, of Judicial Watch, mentioned to us that the government has intercepted ISIS social media posts that indicated a potential attack by crossing the border.This came up in a Sept. 10 Senate Homeland Security Committee hearing. Sen. John McCain, R-Ariz., asked undersecretary for intelligence and analysis Francis Taylor about recent reports of Twitter and Facebook exchanges in which ISIS "urged infiltration" across the U.S.-Mexico border."Yes, sir, there have been Twitter, social media exchanges among ISIL adherents across the globe speaking about that as a possibility," Taylor said, using another name for ISIS, adding: "I'm satisfied that we have the intelligence and the capability at our border that would prevent that activity."This admission energized the discussions in the conservative media. But social media threats should be taken with a grain of salt, said Colin Clarke, an expert on international security at the Rand Corporation, a think tank."ISIS could say 'we would never consider (crossing the border),' and would we take that seriously?" Clarke said. "It's really just one small sliver of evidence."• • •We talked to several national security experts, and each one said they haven't seen credible evidence that ISIS is staging a plot that involves crossing the southern border. "There is a big difference between a theoretical risk or a risk that is worth worrying about," said David Schanzer, director of Duke University's Triangle Center on Terrorism and Homeland Security.The general sentiment among experts was that it's possible that ISIS could sneak through the border — illegal immigrants do it every day. But why would they?Schanzer said that ISIS members who have U.S. passports or visas could enter the country legally via plane. Whereas if they crossed the border illegally, they would run the risk of getting caught.Additionally, ISIS is "overwhelmingly focused" on Iraq and Syria, said Daniel Benjamin, former ambassador at large and coordinator for counterterrorism at the State Department.It's a far stretch to say "we know" with certainty that ISIS is in Ciudad Juárez, so we rate this claim Mostly False.Edited for print. Read the full version at PolitiFact.com.