Tracking a cellphone is easy, especially for the National Security Agency. But can you track a cellphone that's been turned off?

It sounds impossible, but the NSA apparently has been able to track powered-down mobile phones since 2004, as reported by The Washington Post in July 2013.

The Post's mention of this ability was brief. It was buried within a longer narrative regarding the NSA's partnership with the U.S. military's Joint Special Operations Command (JSOC) to track and kill high-profile al-Qaida targets in Iraq:

"By September 2004, a new NSA technique enabled the agency to find cellphones even when they were turned off. JSOC troops called this 'The Find,' and it gave them thousands of new targets," the Post reported.

MORE: NSA Leaks 2013 – A Timeline of NSA Revelations

Hoping for more information, British watchdog group Privacy International in August wrote directly to eight major mobile-phone manufacturers and operating-system providers asking how that could be possible.

So far, Ericsson, Google, Nokia and Samsung have responded — and, as far as they know or can say, it shouldn't be possible to track powered-down cellphones.

All four companies claimed to be unaware of any exploit or vulnerability that would make tracking a powered-down phone possible, since pressing the "off" button on a phone entirely deactivates its network connectivity.

"When a mobile device running the Android Operating System is powered off, there is no part of the Operating System that remains on or emits a signal," Google told Privacy International.

Similarly, Samsung Vice President Hyunjoon Kim wrote: "Without the [mobile phone's] power source, it is not possible to transmit any signal, due to the components being inactive. Thus the powered-off devices are not able to be tracked or monitored by any 3rd party." (You can read Samsung's letter on Privacy International's website.)

Nokia's Chad Fentress had a similar statement, but his phrasing raised eyebrows at Privacy International: "Our devices are designed so that when they are switched off, the radio transceivers within the devices should be powered off." (Nokia's statement is also available on the website.)

Privacy International research officer Richard Tynan told Ars Technica that Nokia's wording, particularly the "should," is suspicious.

"Nokia's wording is very nuanced," Tynan said. "They don't say that transceivers 'are' switched off."

However, both Ericsson and Samsung suggested that it might be possible to place spyware on a phone that would keep some of its network functions active even after users pressed the power button to turn it off.

Without confirmation from the NSA, or access to the documents leaked by former NSA contractor Edward Snowden, it's impossible to tell exactly how, or even if, tracking powered-down mobile phones is possible.

Privacy International is still waiting for responses from Apple, HTC, Microsoft and BlackBerry. The organization plans to reach out to LG, Motorola, Sony and others in the near future.

Email jscharr@techmedianetwork.com or follow her @JillScharr and Google+. Follow us @TomsGuide, on Facebook and on Google+.