The Islamic State has likely radicalized thousands of people in the United States, according to a new report, raising concerns that supporters of the terrorist group could be plotting domestic attacks similar to the recent shootings and bombings in Paris.

The Threat Knowledge Group, an organization led by the counterterrorism experts Sebastian and Katharine Gorka, has compiled a list of 82 individuals in the United States who were affiliated with the Islamic State (also known as ISIS or ISIL) and apprehended by law enforcement officials, including those who traveled or attempted to travel to Iraq and Syria, launched domestic attacks, or participated in recruiting or fundraising.

The Gorkas note in a new report that almost one third of these individuals had plotted attacks against Americans on U.S. soil in the last 18 months. Sebastian is also an adviser to the Department of Defense, while Katharine has authored several publications about the terrorist threat to the U.S. homeland.

Through Islamic State propagandists on Twitter and other social media sites, the terrorist group has been able to attract hundreds more supporters in the United States, they said.

Ali Shukri Amin, a 17-year-old Virginia resident and Islamic State supporter who was sentenced to 11 years in prison in August, used his Twitter account with 4,000 followers to raise funds for the group and encourage friends to join it overseas.

Ahmad Musa Jibril, an Islamist preacher in Dearborn, Michigan, who has also spent time in jail for money laundering and tax evasion, also points to the terrorist group’s online reach. Jibril has more than 38,000 Twitter followers, though he has not tweeted since last July and may have found other methods of communication. He is reported to have a large following among foreign fighters in Iraq and Syria.

"Based on the evidence available, the number of ISIS supporters in the United States measures in the thousands, rather than hundreds," the Gorkas said in their report.

"Whether ISIS will launch an attack on the scale of the Paris attack is unknown, but it is clear that the United States is a primary target for ISIS and that ISIS has the necessary supporters in place and the financial means to carry out such an attack," they continued. "The challenges of screening incoming refugees may further exacerbate the problem."

The Islamic State has displaced al Qaeda as the top threat to the U.S. homeland, the Gorkas said. After examining arrests per month, they found that U.S. police are interdicting 300 percent more Islamic State recruits than al Qaeda supporters.

The Islamic State has been able to attract thousands of foreign fighters and U.S. supporters through its aggressive dissemination of propaganda on social media and urgent ideological and religious narrative.

Religious authorities for the group claim that the "end times" are swiftly approaching, a period when Muslims will defeat invading Christians in Syria in "the final jihad" before the world ends and devout Muslims ascend to heaven. "They are able to persuade many supporters to come fight on the grounds that this is ‘The Final Jihad,’" the Gorkas said.

The Islamic State also focuses on young recruits, who are more vulnerable to radicalization. The report notes that 63 percent of those arrested in the United States for supporting the group were between the ages of 15 and 25.

Islamic State supporters often recruit in clusters in the United States, with friends and family members forming a local jihad network.

"For law enforcement, it suggests that if one person in a community affiliates themselves with ISIS, one can expect to see more, especially among those who are close to the recruit," the Gorkas said.

Still, Islamic State adherents in the United States are vulnerable to capture. Among the 82 cases of U.S. supporters studied by the Threat Knowledge Group, about 60 percent were identified by authorities through their social media posts or a tip from someone close to them. The Gorkas recommended that police build trust in communities so families and residents will feel comfortable turning in those that they suspect are radicalized.

Additionally, they urged law enforcement authorities to work with educators about identifying signs of radicalization among youth, focus more on tracking the Islamic State’s religious authorities online and their propaganda, and develop a more rigorous screening process for refugees from the Middle East.

The State Department recently issued a worldwide travel alert through Feb. 24, 2016, for Americans, urging them to "exercise vigilance when in public places or using transportation." Potential terrorist "attacks may employ a wide variety of tactics, using conventional and non-conventional weapons and targeting both official and private interests," the department said.

"There is a continuing threat from unaffiliated persons planning attacks inspired by major terrorist organizations but conducted on an individual basis," the department added.