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 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.