In the United States, Special Agent is usually the title for a detective or investigator for a state, county, municipal, federal or tribal government who conducts criminal investigations and has arrest authority. An Agent is a federal law enforcement officer with arrest authority but who does not conduct major criminal investigations or who may conduct investigations but does not have arrest authority. Special Agents are distinctly able to do both.
Pursuant to Title 10 of the United States Code §1585, DCIS Special Agents conducting, supervising, or coordinating investigations of criminal activity in programs and operations of the Department of Defence have the authority to execute and serve any warrant or other process issued under the authority of the United States; to make arrests without a warrant for any offense against the United States committed in the presence of that agent; and to make arrests without a warrant for any felony cognizable under the laws of the United States if the agent has probable cause to believe that the person to be arrested has committed or is committing the felony.
Authorization for Special Agents of the Defense Criminal Investigative Service to carry firearms while assigned investigative duties or other duties as the Secretary may prescribe can be found in 10 U.S. Code §1585