Help spread ISIS propaganda on Twitter or Facebook, and you could go to jail. That’s the message the Justice Department sent Monday, as a top official said he is willing to indict people who assist ISIS with its use and production of social media. The announcement raises questions about where the government would draw the line between support for a terrorist group and legally protected free speech.

Provocative tweets, Facebook posts, and grisly online beheading videos have all been key parts of ISIS’s recruitment and propaganda strategy, and one of the hardest elements of the terror group’s rise for U.S. national security agencies and technology companies to combat. And ISIS has attracted supporters online who, while they don’t participate in attacks or killings, endorse the group’s actions and proliferate its message. The Obama administration devoted much of last week to a summit on countering extremism—especially extremism online.

But John Carlin, the assistant attorney general for national security, told a cybersecurity conference in Washington on Monday that officials could try to blunt ISIS’s violent PR operation by essentially trying propagandists as terrorists. He suggested the Justice Department could bring prosecutions under the law against providing material support to a terrorist organization. His remarks were believed to be the first time a U.S. official has ever said that people who assist ISIS with online media could face criminal prosecution.

Carlin was asked at the conference whether he would “consider criminal charges” against people who are “proliferating ISIS social media.”

His answer: “Yes. You need to look at the particular facts and evidence.” But Carlin noted that the United States could use the material support law to prosecute “technical expertise” to a designated terrorist organization. And spreading the word for ISIS online could count as such expertise.

Carlin didn’t expand upon his remarks, which were largely devoted to the Justice Department’s efforts to counter cyber espionage. A spokesperson for Carlin didn’t respond to a request for further comment.

But legal experts have questioned the use of material support applied to speech that is normally protected by the First Amendment, indicating that Carlin’s idea rested on controversial grounds.

“It’s highly doubtful that the material support statute could be constitutionally applied to the simple promotion of a terrorist group on Facebook,” David Greene, the civil liberties director and a senior staff attorney at the Electronic Frontier Foundation, told The Daily Beast. In a 2008 case, Holder v. Humanitarian Law Project, the Supreme Court upheld the material support statute in the face of a First Amendment challenge, Greene said.

“But in that case, the court made clear that the statute was constitutional because the government sought to apply it to the provision of advice that involved ‘specific skill’ or was derived from ‘specialized knowledge,’” Greene added. “But the law could not be applied if the publisher’s speech imparted only ‘generalized or unspecialized knowledge.’ In fact, in Holder, the government argued that it could not and would not apply the law to independent advocacy for or expression regarding terrorist organizations.”

The most notable and controversial case of prosecuting speech linked to terrorism involved a man who also attempted to join an al Qaeda training camp in Yemen and didn’t focus on actions taken exclusively online.

In 2012, the United States won a conviction against Tarek Mehanna, a Massachusetts pharmacist who tried unsuccessfully to fight with al Qaeda in Iraq but also translated a number of jihadist tracts and videos into English so that they could be distributed online and encourage others to join the terror group.

The key issue in Mehanna’s case was whether his translations were an expression of sympathy for al Qaeda’s ideology—which would be protected—versus work that he undertook to support the terrorist group or assist its operations. A jury convicted Mehanna under the material support statute, and he was sentenced to 17½ years in prison.

But the courts didn’t fully settle the consitutional questions in the case, and it’s not clear how the material support law could similiarly be used against people who engage in or support ISIS’s social media campaigns.

“The problem boils down to this: The government equated rhetorical support with ‘material support,’” Alex Abdo, a staff attorney with the American Civil Liberties Union, wrote at the time. “What makes the government’s theory so dangerous to our freedom of speech is that it threatens to dismantle the barrier that the Supreme Court has erected ‘between words and deeds, between ideas and conduct.’ That barrier prevents the government from censoring speech based solely on its tendency to persuade...”

Social media has presented national security officials trying to prevent terrorist attacks with a new challenge, Carlin said. He called social media platforms like Twitter and Facebook “essentially a free form of communication that you can use to plot and plan,” and noted that governments would spend billions of dollars to deploy a command and control system that was battle-ready, while ISIS has turned to common Internet technologies. U.S. counterterrorism officials say the group has been careful to use social media and chat applications in a way that avoids detection by American surveillance agencies, notably the NSA.

Carlin made his remarks at a conference sponsored by the think tank New America during a live interview with Peter Bergen, a CNN analyst and New America director. (Full disclosure: I’m a fellow at New America and spoke at the same conference.)

Another senior U.S. national security official also warned that terrorists were employing everyday technology and urged Congress to find a “legal framework” under which the government could monitor global communications more easily.

NSA Director Adm. Michael Rogers said he agreed with public statements by FBI Director James Comey that the use of encryption, particularly on popular products like the iPhone 6, put the government at risk of not being able to monitor terrorists and spies. Rogers said lawmakers should come up with a solution for ensuring government access to encrypted communications, a plan that many technologists and civil libertarians have decried as a “backdoor” to spy on people around the world.

In a tense exchange, Alex Stamos, the chief information security officer for Yahoo, asked Rogers whether what he was really advocating was “building defects” into encryption technology.

“That would be your characterization,” Rogers replied, to nervous laughter from the audience.

The back and forth between Stamos and Rogers underscored how divisive the relationship between Silicon Valley and Washington has become in the wake of disclosures by former NSA contractor Edward Snowden, who revealed that the spy agency has broken into foreign data centers operated by Yahoo and Google in order to steal information on their customers.

Last week, The Intercept reported that the NSA had also spied on employees of a Dutch SIM-card manufacturer in order to steal encryption keys used on billions of the chips, illustrating how far the agency will go to defeat encryption technology. The operation gave the NSA and its British counterpart the ability to spy on people around the world without having to inform or consult technology companies or governments.

Stamos asked, “If we’re going to build defects, backdoors, or golden master keys for the U.S. government, do you believe we should do so—we have about 1.3 billion users around the world—should we do so for the Chinese government, the Russian government, the Saudi Arabian government, the Israeli government, the French government? Which of those countries should we give backdoors to?”

“The way you framed the question isn’t designed to elicit a response,” Rogers said. Asked by an audience member to elaborate on the Intercept report about the NSA stealing encryption keys, the spy chief replied, “I am not going to chase every allegation out there. I don’t have the time.”