A terror suspect who was shot dead by authorities Tuesday in Boston was plotting to behead police officers, a law enforcement source told Fox News.

FoxNews.com reports

Authorities in Massachusetts arrested a man in connection with a counterterror investigation late Tuesday, hours after another man under surveillance by the Joint Terrorism Task Force was shot and killed after he refused to put down a military-style knife while approaching two officers.

"We believe the intent was to behead a police officer," one official told The Boston Globe. "We knew the plot had to be stopped. They were planning to take action Tuesday."

Authorities identified the deceased man as 26-year-old Usaama Rahim, who was shot outside a CVS Pharmacy in Roslindale, Mass. at approximately 7 a.m. Tuesday.

Authorities said yesterday that FBI and Boston police investigators approached the man to ask him some questions. He then pulled out a black military-style knife and lunged at investigators.

Police Commissioner William B. Evans said the officers gave the suspect several commands to put down the weapon before they opened fire.

The suspect was transported to a local trauma hospital, but did not survive his wounds.

Rahim's family, however, has disputed the police version of the confrontation.

Fox News chief intelligence correspondent Catherine Herridge reported the latest (above) this morning on "America's Newsroom." 

She said intelligence sources told her that Boston police were concerned about an ISIS-style execution of police officers.

A second man, David Wright, was arrested in Everett, Mass., and that is believed to be connected to the Roslindale incident. 

Earlier on the show (video below), Chairman of the House Homeland Security Committee Mike McCaul (R-Texas) expressed his concern that "terrorism has gone viral." 

McCaul said recruiters in Syria are sending calls to arms to followers in the United States, essentially trying to activate sleeper cells. 

Herridge said it's still unclear whether these suspects were being directed by ISIS. At a Homeland Security Committee hearing today, lawmakers will be discussing the online radicalization.

She said investigators are concerned that ISIS is directing sympathizers to encrypted forms of communication, allowing them to "go dark."