That's according to "former officers" interviewed by the Wall Street Journal ahead of privacy advocate Christopher Soghoian's presentation at hacker-conflab Black Hat later today.
The FBI's Remote Operations Unit has been listening in to desktop computers for years, explains the paper, but mobile phones are a relatively new target.
It would never work with tech-savvy suspects, though: suspects still need to infect themselves with the malware by clicking a dodgy link or opening the wrong attachment. This is why computer hackers are never targeted this way – they might notice and publicise the technique, said the "former officers", who noted that in other cases it had proved hugely valuable.
Such actions do require judicial oversight, but if one is recording activities rather than communications, the level of authorisation needed is much reduced. A US judge is apparently more likely to approve reaching out electronically into a suspect's hardware than a traditional wiretap, as the latter is considered a greater intrusion into their privacy.
Gaining control of that hardware still requires a hole to crawl through; ideally a zero-day exploit of which the platform manufacturer is unaware.
The WSJ cites UK-based lawful spook spyware supplier Gamma International as selling such exploits to the Feds. The company was recently in the news after allegations that it was also supplying dodgy governments with kit - allegedly including malware disguised as the Firefox browser.
Given the convergence of mobile and desktop, it's no surprise to see desktop techniques being applied to mobile phone platforms by both hackers and law enforcement agencies.
The usual techniques of not opening unknown attachments or unsigned downloads should protect you against the FBI, just as it would against any spear-phishing attempt. But then again, if you know that, they probably wouldn't try using it against you. ®