A photo taken on Sept. 11, 2015, in northern Iraq shows smoke rising in the distance behind an Islamic State group flag and banner. (Marwan Ibrahim/AFP/Getty Images)

The Justice Department has charged at least 60 individuals this year with terrorism-related crimes, an unprecedented number that officials attribute to a heightened threat from the Islamic State and the influence of social media on potential recruits.

Last week alone, prosecutors charged three people and convicted two others on terrorism-linked charges. One of those charged was Enrique Marquez Jr., a friend and neighbor of Syed Rizwan Farook, the male gunman who was killed in a firefight with police in the aftermath of the San Bernardino, Calif., shooting rampage this month. That terrorist attack killed 14 people.

A second was Mohamed Elshinawy, a Maryland man accused of receiving at least $8,700 from the Islamic State overseas and planning to use the money to carry out attacks in the United States. He told prosecutors that a childhood friend had connected him through social media with an Islamic State operative.

The third was Jalil Aziz, a Pennsylvania man who was arrested for allegedly providing material support to the Islamic State, by spreading its propaganda on social media and for seeking to help the group’s supporters travel to Syria to fight. Aziz also encouraged other Islamic State supporters he communicated with to use U.S.-based encrypted messaging applications, prosecutors said.

“The common connection we’re seeing is — in almost every case — a tie to social media,” said John P. Carlin, assistant attorney general for national security, at a conference last month hosted by the news site Defense One. He also pointed out that many of the cases involve young people, who are at ease building relationships online.

More than 55 percent of those charged are under 25 years old. Most troubling, Carlin said, about one-third are 21 or younger.

“That’s not the same age demographic that we saw with al-Qaeda,” he said in a discussion at the Atlantic Council last month.

Islamic State supporters inside the United States, inspired by its leaders’ calls to attack where they reside, are increasingly plotting or attempting to carry out attacks domestically, officials said. In 2015 alone, prosecutors brought more than 15 cases against such individuals.

And for the first time, prosecutors charged a suspect with hacking and supporting terrorism — a form of cyberterrorism that uses the tools of cybercrime in support of a terrorist organization. Ardit Ferizi, a citizen of Kosovo, was detained in Malaysia on a U.S. provisional arrest warrant.

The charges, announced in October, accuse Ferizi of using online means to steal the personal data of U.S. service members and passing it to Junaid Hussain, an Islamic State hacker.

Hussain, a British citizen who was killed in August in a drone strike in Syria, had posted links on Twitter to the names, email addresses, locations and phone numbers of 1,354 U.S. military and other government personnel. He included a warning that Islamic State soldiers would “strike at your necks in your own lands!”

The Washington Post gives an insider's look at the Islamic State's propaganda machine and its influence throughout the world. (Jason Aldag/The Washington Post)

The FBI has opened investigations in all 50 states, and the cases cross ethnic and geographic lines, officials said.

FBI Director James B. Comey testified to Congress this month that he had “no reason to believe” that the Islamic State had established cells in the United States. Rather, he said, “they’re trying to motivate people already in the United States to become killers on their behalf. And they would very much like to be . . . the leader in global jihad, to send people here to conduct attacks. It’s that second piece we have not seen yet.”

The period leading up to the July 4 holiday was particularly busy for the FBI and Justice Department. There was a “flood of information” pointing to Islamic State attempts to identify individuals inside the United States with extremist views to motivate them to launch attacks, said Nicholas J. Rasmussen, director of the National Counterterrorism Center, speaking at the Defense One summit. It was “taxing our resources,” he said.

Comey told reporters that in the month leading up to July 4, the FBI arrested more than 10 people who were suspected of ties to the Islamic State. All, he said, were products of online recruiting and radicalization efforts. However, he said, not all were engaged in active plots.

In most of the cases brought in the past two years, prosecutors relied on a statute that enabled them to bring charges of providing material support to a terrorist group without requiring a link to a specific attack. Such preventive prosecution became a priority after the Sept. 11, 2001, attacks, and it is an important strategy again, said Robert M. Chesney, a law professor at the University of Texas who closely follows counterterrorism cases.

But it hasn’t prevented all attempted attacks. In May, two men with evident sympathies for the Islamic State opened fire on officers outside a Garland, Tex., auditorium in which cartoon images of the prophet Muhammad were featured. The assailants were shot and killed.

In July, a gunman fired on two military facilities in Chattanooga, Tenn., killing four Marines. A Navy sailor died later from his wounds. In December, Comey said the attacks were motivated by “foreign terrorist organization propaganda.”

And on Dec. 2 in San Bernardino, Farook and his wife, Tashfeen Malik, carried out the worst mass terrorist attack on U.S. soil since 9/11. The couple were killed in the ensuing shootout, but not before Malik had posted a pledge of allegiance on her Facebook page to the Islamic State leader, Abu Bakr al-Baghdadi.

In the end, Carlin said, prosecutions alone are not the measure of success. “Success will be figuring out a way to counter the strategy of the tactics [of the Islamic State], so they are no longer reaching young people in the United States” with a message that radicalizes them.