The FBI has been rounding up more potential “lone wolf” terrorists, congressional leaders and the Justice Department say, in response to the perception of a mounting threat of domestic attacks inspired by the Islamic State.

Since the thwarted attack on a “Draw Mohammed” conference in Garland, Texas, on May 3, the Justice Department has announced the arrests of 10 individuals it says were inspired by and supporting the Islamic State. The lawmakers say there have been more arrests that have not yet been announced.

They say the FBI has shifted its approach toward arrests rather than keeping suspects under surveillance, and is also targeting individuals thought to be planning attacks in the U.S., unlike the bureau’s past focus on volunteers preparing to join Islamic State’s fight abroad.

“Lately, we have seen an uptick in the number of arrests of ISIL followers who were planning violent acts in our homeland,” said John Carlin, the assistant attorney general for national security. “ISIL, differing from some other foreign terrorist organizations, has demonstrated that they see value in mobilizing sympathizers anywhere in the world.”

The spate of arrests comes in response to what congressional leaders and the Justice Department say is a mounting threat that radicalized Americans will attempt low-tech, lone wolf attacks in the near future. Lawmakers see the changes as necessary because the Islamic State uses social media so effectively to radicalize Americans and because the group is getting better at using encryption to shield its communications with new recruits.

The shift has downsides. An emphasis on arrests rather than surveillance limits intelligence gathering. Arresting suspected recruits before they’ve acted makes prosecuting them more difficult. It could also violate the First Amendment right to free expression, if terrorist sympathizers are treated as terrorist supporters.

The recent arrests are “an indication that the increased number of threads of threats … is at the highest level that most of us have seen since 9/11,” Chairman Richard Burr of the Senate Select Committee on Intelligence told us in an interview.

His comments track with those of his House counterpart Devin Nunes, who said earlier this week that the country is at a higher risk of terrorism than ever before.

In almost all of the FBI’s recent terrorism arrests, the suspects are charged with providing material support for a terrorist group, a catch-all charge that can mean providing travel documents and cash to would-be-terrorists or trying to recruit new adherents.

Burr vigorously defended the approach, telling us, “There is a sufficient case there to be made with every one of them.”

Some watchdogs see the “material support” charge as prone to overuse. Hina Shamsi, the director of the American Civil Liberties Union’s national security project, told us that the statute has been interpreted expansively by the Justice Department since 9/11, and applied especially to Muslims. She said that now the charge of providing material support to a terrorist group “seems increasingly targeted at people’s online expression or association, without sufficient connection to actual wrongdoing.”

One hard case may end up being that of Ali Shukri Amin, a 17-year-old from Manassas, Virginia, who pleaded guilty to providing material support to the Islamic State through his use of social media. Part of Amin’s guilty plea acknowledged using his Twitter account to provide potential Islamic State recruits instructions on how to use the virtual currency bitcoin and to help recruits travel to Syria.

In other recent cases, the Justice Department accuses Americans of planning to do violence directly. Justin Nojan Sullivan, a 19-year-old from Morgantown, North Carolina, was arrested in his home on June 19. Carlin said in a statement this week that Sullivan would be charged with “planning assassinations and violent attacks in the United States.” The FBI also alleges that Usaamah Abdullah Rahim, the 26-year-old killed by an FBI agent in Boston on June 2, was involved with two other local men in planning a killing spree inside the U.S.

Dianne Feinstein, the top Democrat on the Senate committee, told us that the intelligence reporting about potential domestic attacks by Islamic State-inspired lone wolves has been at a high pitch over the last two months.

“There’s been a lot of intelligence that showed they are going to try to attack police, military, as well as others,” she said. “Nothing is specific; that’s the problem. But I think it’s a correct thing to take it seriously and do everything you can to prevent one from happening.”

Other top Democrats noted the need to balance preventing attacks against the need to gather more information about the Islamic State and its presence in the U.S., as well as build cases that can lead to convictions.

“It’s a mixed bag because obviously you don’t want to wait too long,” said Rep. Adam Schiff, the top Democrat on the House Intelligence Committee. “On the other hand if you act too early, you may not have a prosecutable case, like if people are just expressing some kind of affinity for the social media.”

The calculation by intelligence and law enforcement agencies has changed in part, he said, because the Islamic State is more interested in inspiring simple, one-off, low-level attacks than groups like al-Qaida, which has preferred dramatic, high-casualty operations.

Also, there’s lots of evidence that recent lone wolf attacks in Canada, Australia and France involved some level of social media incitement, if not direct contact with Islamic State social media operatives.

“That has caused our law enforcement to go back over their list of people who had come to our attention and perhaps not wait until they act out and attempt to nip these cases in the bud,” Schiff said.

Several U.S. officials and members of Congress have discussed openly how the Islamic State uses social media to lure young people to commit jihad. But monitoring such activity is complicated by the Islamic State’s use of encryption software and tools that erase records of a conversation.

Rep. Mike McCaul, the Republican chairman of the House Homeland Security Committee, has called these encrypted communications “dark spaces” because the FBI cannot eavesdrop on these communications. At a June 3 hearing before McCaul’s committee, the bureau’s assistant director for counterterrorism, Michael Steinbach, acknowledged that the FBI could not monitor much of the Islamic State’s encrypted chats.

One senior U.S. intelligence official told us that the inability to listen in on these conversations — where law enforcement officials suspect more detailed recruitment occurs – – has caused a panic inside the U.S. intelligence community.

Blocked from some conversations, the FBI is taking part in others — perhaps even impersonating Islamic State social media accounts. After the bureau arrested Amir Said Abdul Rahman al- Ghazi, a 38-year-old from North Olmstead, Ohio, the announcement said al-Ghazi “communicated with individuals he believed to be members of ISIL in the Middle East and took steps to create propaganda videos for ISIL.” The FBI is also charging Abdul Rahman with selling marijuana.

Despite the increase in arrests and the government’s more aggressive actions to quash online incitement at earlier stages, many in Washington believe that the threat from the Islamic State is only growing and that there’s just no way America’s law enforcement and intelligence agencies can keep pace. FBI Director James Comey has said the group is recruiting in all 50 states.

“There are more of these guys to follow than we can possibly follow,” Sen. Lindsey Graham told us. “It’s just a matter of time before one of them penetrates the net.” He advocates deploying 10,000 U.S. troops to Iraq to fight the Islamic State at its source, and he faults Obama for not doing so.

Democrats acknowledge that the problem is only getting worse, but they disagree with hawks like Graham.

“There’s merit to the argument that the longer that ISIS carries on, the more we are going to have to deal with this continual threat of ISIS social media inspired radicalism,” Schiff said. “But that doesn’t mean we have to be sending in the Marines.”

Josh Rogin, a Bloomberg View columnist, writes about national security and foreign affairs. Eli Lake, a Bloomberg View columnist, writes about politics and foreign affairs.