A Boston police officer has shot and killed a man who had been under surveillance by the FBI's Joint Terrorism Task Force, ABC News has learned.

The FBI had been tracking the man for several weeks, and authorities are looking into whether he may have been radicalized by ISIS propaganda online, law enforcement sources told ABC News.

An officer and an FBI agent approached the suspect at about 7 a.m. today in the parking lot of a CVS in Roslindale, Massachusetts, police said. The suspect then pulled a "military-style knife," prompting numerous commands from the officer and FBI agent to drop the weapon. But when the suspect refused, both the officer and FBI agent opened fire, officials said.