who has the authority to grant a pardon or clemency
Pardon In the United States, the pardon power for offences against the United States is granted to the President of the United States under Article II, Section 2 of the United States Constitution which states that the President "shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment". The U.S. Supreme Court has interpreted this language to include the power to grant pardons, conditional pardons, commutations of sentence, conditional commutations of sentence, remissions of fines and forfeitures, respites, and amnesties.[27]
Capital punishment by the United States federal government It can be handed down for treason, espionage, murder, large-scale drug trafficking, or attempting to kill a witness, juror, or court officer in certain cases; but all inmates currently under federal death sentences were condemned for aggravated murder. Executions performed by the federal government are infrequent compared to those performed by state governments.[1][2]