WASHINGTON (AP) — Two people who prosecutors say were motivated by white supremacist ideology have been arrested on charges that they used the social media messaging app Telegram to encourage acts of violence against minorities, government officials and critical infrastructure in the United States, the Justice Department said Monday.

The defendants, identified as Dallas Erin Humber and Matthew Robert Allison, face 15 federal counts in the Eastern District of California, including charges that accuse them of soliciting hate crimes and the murder of federal officials, distributing bombmaking instructions and conspiring to provide material support to terrorists.

Humber, 34, of Elk Grove, California, and Allison, 37, of Boise, Idaho were arrested Friday. It was not immediately clear if either had an attorney who could speak on his behalf.

The indictment accuses the two of leading a transnational group known as Terrorgram that operates on Telegram and espouses white supremacist ideology and violence to its follows.

Justice Department officials say the men used the app to transmit bomb-making instructions, to distribute a list of potential targets for assassination — including a federal judge, a senator and a former U.S. attorney — and to celebrate people accused in prior acts or plots of violence, such as the stabbing last month of five people outside a mosque in Turkey and the July arrest of an 18-year-old accused of planning to attack an electrical substation to advance white supremacist views.

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“I think it would be difficult to overstate, the danger and risks that that this group posed,” Assistant Attorney General Matthew Olsen, the Justice Department’s top national security official, said at a news conference.

The pair’s exhortations to their follows to commit violence included statements such as “Take Action Now” and “Do your part,” according to an indictment unsealed Monday.

“Today’s action makes clear that the department will hold perpetrators accountable, including those who hide behind computer screens, in seeking to carry out bias-motivated violence,” said Assistant Attorney General Kristen Clarke, the department’s top civil rights official.

The founder and CEO of Telegram, Pavel Durov, was detained by French authorities last month on charges of allowing the platform’s use for criminal activity. Durov responded to the charges by saying he shouldn’t have been targeted personally.