SEATTLE >> Federal prosecutors have dropped child pornography charges against a Washington teacher after the U.S. Justice Department refused to disclose information about a software weakness it exploited during an investigation last year.

Charges against Vancouver, Washington, teacher Jay Michaud in U.S. District Court in Seattle were dismissed Monday.

In 2015, Michaud was arrested and accused of downloading child pornography. During the child porn investigation, the FBI allowed a secret child porn website on the largely anonymous Tor network to run for two weeks while it tried to identify users by hacking into their computers.

The child porn website, called Playpen, operated on Tor, which provides users anonymity by routing their communications through numerous computers around the globe, and it had more than 150,000 members. The Tor browser is based on Firefox. While the network is used for various reasons — including circumventing free-speech restrictions in some parts of the world — it has also provided sanctuary for child pornography, drug trafficking and other criminality.

After arresting Playpen’s operator in Florida in early 2015, the FBI let the website continue running for two weeks while trying to identify users, a move the agency said was necessary to apprehend those posting and downloading images of children being sexually abused. Defense attorneys criticized the tactic as unethical.

A magistrate in Virginia issued a search warrant allowing the agency to deploy what it calls a “network investigative technique”: code that prompted the computers that signed into Playpen to communicate back to the government certain information, including IP addresses, despite the anonymity normally afforded by Tor.

The FBI then obtained further warrants to search suspects’ homes. At least 137 people were charged. Defendants have challenged the FBI’s hacking on numerous grounds.

A federal judge in Washington state threw out the government’s evidence against Michaud last year, saying that unless the FBI detailed the vulnerability it exploited, the man couldn’t mount an effective defense.

The DOJ said previously the information is not relevant. Defendants have been offered or provided all the evidence they need, including limited source code and data streams showing what the program did, the FBI has argued.

Michaud’s lawyer, Colin Fieman, said in an email to The Associated Press that they are relieved and grateful his case is done but that many unanswered questions remain about the FBI’s investigation, known as Operation Pacifier.

“Mr. Michaud maintained his innocence from the outset, and the dismissal is a result of the FBI’s overreaching and misuse of its computer hacking capabilities, including its operation of the world’s largest child pornography web site and attacks on computers in over 120 countries,” Fieman said. “It remains to be seen whether the FBI will ever be held fully accountable for those aspects of its investigation that put core privacy rights at risk and violated common standards of decency when it comes to how law enforcement agencies do their job.”

A school district spokeswoman says Michaud hasn’t returned to work, KGV-TV reported.