"We've got definitive proof that someone is here in this area is operating some sort of intercept device," Turner said as his car rolled down Pennsylvania Avenue. "Whether that's just tracking people or listening to calls, we can't say."
Turner said he couldn't say who was trying to crack into his phone, but he said his best guess was that it was not U.S. government surveillance. Instead, he suspected the ESD devices had stumbled on evidence of corporate espionage in action, possibly targeting large law firms or corporate board of directors meetings in office buildings nearby.
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"The information that we've gotten during this research session basically says that they're looking at everybody, but they're picking a couple of select people to intercept," Turner said. "On regular consumer devices, people are totally at risk."
Turner said the attackers trying to crack into his phone were operating a device called an IMSI catcher, which is an eavesdropping device that mimics a cellphone tower. Mobile phones constantly scan for the nearest available cellphone tower, but when they connect with the IMSI catcher, they can be fooled into transmitting information as if to a real tower. In reality, an attacker has gotten access to that phone, and can use malware to prevent the phone from giving any indication to its user that it has been hacked.
ESD installs interceptor-detecting software written by the German firm GSMK onto Samsung mobile phones running an Android system and resells the phones to its customers for more than $3,000 each. The company says it sells the phones to people and companies who handle sensitive materials on their phones and are paranoid enough to pay a hefty fee to protect themselves.
The claims made by ESD, Goldsmith and Turner cannot be independently verified by CNBC. ESD said it is the only company that makes a commercially available device designed to detect cell phone interception.
Not all cellular experts are convinced by ESD's findings. The company, after all, has a vested interest in selling its products. "The sightings are technologically possible," said cryptographer and security researcher Karsten Nohl, "but unrealistically frequent in the recent reports."
Other experts, though, say ESD is on the right track. Asked whether ESD's claims are credible, Joshua Marpet, a security researcher at the data security firm GuardedRisk, said: "Oh, Lord, yes, it's realistic." He added, "they can't tell you that you are connected to a rogue tower or IMSI catcher. What they can tell you is that the tower your phone is trying to connect to does not have a correct ID, is numbered badly, has turned off encryption, or in some other way, is simply 'off.' "
Alan Butler, senior counsel at the Electronic Privacy Information Center, a public interest group focused on civil liberties, said the techniques used by ESD are generally plausible. "This sounds like a pretty solid technical way to detect such a device," Butler said. "Plans for a device that would be in effect an 'IMSI catcher-catcher' have been out there for a while."
The Electronic Privacy Information Center has been engaged in a long-running dispute with the FBI about law enforcement's own use of cell phone interceptors, which privacy advocates worry could be abused because of their wide-ranging powers. "The FBI has been very tight-lipped about the use of this technology, and has been for many, many years," Butler said. "But it is a pretty significant piece of their surveillance toolkit."
CNBC contacted the FBI and Department of Justice to ask if there is any law enforcement reason why cell phone interceptors would be operating in downtown Washington.
"We cannot comment on their credibility or their claims," said Andrew Ames, a spokesman for the FBI's Washington field office. "Additionally, we do not comment on whether we employ particular means or methods as part of investigations."
Intercepted cell phone calls have caused trouble in Washington before. In February, an unknown person intercepted a call between Assistant Secretary of State Victoria Nuland and the U.S. ambassador to the Ukraine in which Nuland used a vulgar expletive to refer to the European Union during the unfolding crisis in Ukraine.
A person who sounds like Nuland can be heard on the tape saying "F--- the EU."