Who is America’s front line of defense against Terrorism in the Homeland

Obviously Americans are. The “see something, say something” program works well and is effective, but when you say something who answers the call?
The FBI Joint Terrorism Task Force is the investigative body, or group responsible for the investigation of terrorism.
The task forces consist of highly trained and committed investigators from federal, state, and local law enforcement agencies as well as analysts, linguists, and occasionally regulatory agencies, all combined to investigate terrorism and related offenses. Most large metropolitan areas have a JTTF operating across all 56 field offices of the FBI.
The first JTTF was established in 1980 based in New York City.