The FBI arrested a third man Thursday in connection with a plot to attack and behead a free speech activist and Boston police officers.

The agency's Rhode Island Joint Terrorism Task Force arrested Nicholas Rovinski in Warwick, R.I. shortly after 8 p.m. local time. FBI spokeswoman Kristen Setera told Fox News that Rovinski was arrested without incident and referred further questions to the U.S. Attorney's office in Boston.

The Boston Herald reported that Rovinski is scheduled to appear in U.S. District Court in Boston Friday, where the charges against him will be announced.

Investigators searched Rovinski's home in Warwick last week after members of the Joint Terrorism Task Force shot and killed Usaama Rahim in Roslindale, Mass. June 2. Authorities believe Rahim had originally planned to behead Pamela Geller, the activist behind last month's "Draw Muhammad" competition in Garland, Texas, before turning his sights to Boston police officers.

Social media accounts linked to a Nicholas Rovinski of Warwick, R.I. that were reviewed by Fox News displayed links to radical Islamist material. A YouTube page included a link to a video of Sheik Omar Bakri, a Syrian cleric believed to have radicalized the men who attacked and beheaded British solider Lee Rigby on a London street in 2012. 

The accounts also included a how-to guide to make machetes without power tools, and audio chants known as nasheeds bearing the ISIS. Investigators believe that Rovinski encouraged Rahim to launch the attack, and may well be the digital link to ISIS overseas.   

Court documents filed last week allege that Rahim traveled to Rhode Island to discuss the plot with his nephew, David Wright, and a third man, believed to be Rovinski. Wright was arrested hours after Rahim's death and charged with conspiracy with intent to obstruct a federal investigation. His next hearing is set for June 19.

Prosecutor Stephanie Siegmann said Wright posed a serious risk of fleeing or obstructing justice if not held pending the hearing. Wright's attorney, Jessica Hedges, denied that, saying he has deep roots in the Boston area and an "incredibly loving and supportive family."

Hedges urged the government to be "as transparent as possible" and "abide by the law" as it investigates this case, saying "we have serious concerns about that already."

The FBI said Rahim, who had previously discussed beheadings, bought three fighting knives and a sharpener on or before May 26 and he told Wright he would begin trying to randomly kill police officers.

The task force of FBI agents and Boston police officers, faced with an imminent threat, confronted Rahim on a sidewalk and fatally shot him when he refused to drop his knife, authorities said.

Rahim's family has disputed investigators' version of events. They said the blurry surveillance video showing the terror investigators fatally shooting him doesn't show him brandishing a weapon or approaching officers aggressively. They said he was not the initial aggressor and did not appear to be breaking any laws as he walked toward a bus stop on his way to work.

Fox News' Catherine Herridge and The Associated Press contributed to this report.