In a surprising and worrying move, the FBI has dropped its case against a man accused of downloading child sex abuse images, rather than reveal details about how they caught him.

Jay Michaud, a middle school teacher in Vancouver, Washington, was arrested in July last year after visiting the Playpen, a dark web meeting place tens of thousands of perverts used to swap mountains of vile underage porn.

Unbeknown to him at the time, the FBI were, for about a fortnight, running the site after taking over its servers, and managed to install a network investigative technique (NIT) on his computer to get his real public IP address and MAC address. The Playpen was hidden in the Tor anonymizing network, and the spyware was needed to unmask suspects – about 1,300 public IP addresses were collected by agents during the operation.

According to the prosecution, a police raid on his home revealed a substantial hoard of pictures and video of child sex abuse on computer equipment. But now, guilty or not, he's now off the hook after the FBI filed a motion to dismiss its own case [PDF] late last month.

Why? Because Michaud's lawyer insisted that the FBI hand over a sample of the NIT code so it could be checked to ensure that it didn't breach the terms of the warrant the FBI obtained to install the malware, and to check that it wouldn't throw up any false positives.

US District Judge Robert Bryan agreed, saying that unless the prosecution turned over the code, he'd have to dismiss the charges. The FBI has since been arguing against that, but has now decided that it's better to drop the case than reveal its techniques.

The Playpen affair has proved to be a legal minefield in more ways than one. For a start, the admission that the FBI had been distributing such images and videos online troubled many. But the agency also only sought a single warrant to distribute its NIT internationally, which may have been illegal at the time.

That's no longer the case, since a change in Rule 41 of the Federal Rules of Criminal Procedure was nodded through by the US Supreme Court and came into effect on December 1 last year. Judges in Playpen cases – there have been hundreds of prosecutions similar to Michaud's lined up by the Feds – haven't always agreed that the FBI had the right to introduce evidence gathered without a local warrant.

In the past the FBI has dropped cases rather than reveal their investigation techniques, particularly with its cellphone-tracking Stingray equipment. But those were minor cases – nothing so serious as child abuse. ®