The Signal Messenger app is displayed on a smartphone in Hong Kong, China.
Roy Liu | Bloomberg | Getty Images

Federal investigators say they accessed encrypted Signal messages sent in the lead-up to the Jan. 6, 2021, riot on the U.S. Capitol, and used them as evidence to charge the leader of the Oath Keepers, an extremist far-right militia group, and other defendants in a seditious plot.

In a new legal complaint made public on Thursday, The Department of Justice alleges the defendants conspired to forcefully oppose the transfer of power between then-President Donald Trump to Joe Biden, including by trying to take control of the U.S. Capitol.

The complaint references numerous messages sent on Signal, an end-to-end encrypted messaging app, raising questions about how authorities accessed them and recalling a long-standing point of tension between the law enforcement community and tech industry. Encryption scrambles messages so that nobody can read them except the intended recipients — including the platform hosting the messages.

It’s not clear how investigators gained access to the messages. Representatives for Signal, the Department of Justice, and Federal Bureau of Investigation did not immediately respond to CNBC’s requests for comment.

One possibility is that another recipient with access to the messages handed them over to investigators. The complaint references group messages run on the app, so it’s possible another participant in those chats cooperated.

Encryption has been a point of controversy between investigators and tech companies for years. While law enforcement worries that criminals will exploit encrypted technology to hide wrongdoing, tech companies like Apple have argued that it’s an important tool for privacy. Law enforcers have in the past tried to get tech companies to open their devices to assist in investigating serious crimes, but companies like Apple argue that if they break encryption for U.S. investigators, it will jeopardize the entire system and potentially leave room for foreign adversaries to exploit weaknesses.

The issue gained particular prominence in 2015, when Apple refused to break the encryption of a suspect’s iPhone in the wake of a mass shooting in San Bernadino, Calif. After a tense stand-off, investigators were eventually able to break the encryption themselves anyway.

But some law enforcers have said newer security features on iPhone software now makes it harder for them to technically access those devices, even if they are able to obtain a warrant.

The issue came up again under the Trump administration, including when Meta, then known as Facebook, announced plans to stitch together all of its messaging services and encrypt them from end-to-end. Law enforcers said the plans would hamper their ability to clamp down on child sexual abuse material on the platform.

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