Understanding the use of the death penalty in the United States &ndash which often is determined by geographic location -- requires an understanding of our federal system.

Despite an enormous increase over the past generation in the amount of federal law enforcement, the vast majority of law enforcement continues to occur at the state and local level. For example, the Federal Bureau of Investigation is the largest federal law enforcement agency, yet it has only about one third as many sworn law enforcement officers (“special agents”) as does the New York City Police Department.

Under the U.S. federal system, the fifty state governments must not act in a way which violates the U.S. Constitution. Yet, the federal government is one of limited powers, and “[t]he powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.” Even in areas in which the federal government may act, its powers are not always exclusive to that of the states. Foreign policy, for example, is exclusively the province of the U.S. Government, but narcotics control is a power shared by the federal and state governments, while routine assaults lie solely within the purview of the states. Rights granted to individuals by the U.S. Constitution generally are minimum protections from government. Thus, the U.S. Congress and the various state governments remain free to provide additional protections to their citizens.

The Fourth Amendment to the U.S. Constitution prevents unreasonable seizures of a person. The Fifth Amendment prohibits deprivation of life, liberty, or property, without due process of law. The Eighth Amendment forbids cruel and unusual punishment. None of these directly authorizes death as a punishment for crimes they only forbid any punishment that is unreasonable, imposed without due process or is cruel and unusual. As currently interpreted by the United States Supreme Court, capital punishment does not necessarily violate these or any other provision of the U.S. Constitution. On the other hand, nothing in the U.S. Constitution requires the imposition of death as a penalty for certain crimes. Thus, both the U.S. Congress as well as each of the fifty state legislatures constitutionally may choose to forbid capital punishment. Or, consistent with the U.S. Constitution, each of them may enact a death penalty for a certain crime, as long as the sentence is not imposed or executed unreasonably, without due process, or in a cruel and unusual manner.

Thirty-two states have the death penalty. Within those thirty-two exist great variations in the crimes eligible and the procedures for imposing capital punishment, as well as the method and rate of executions. In 2012, twenty-three of those states did not execute anyone, whereas Texas executed 15 convicts and Arizona, Oklahoma, and Mississippi each executed six. Eighteen states plus the District of Columbia have abolished capital punishment. Thus, a crime may generate a death sentence or not depending upon which side of an invisible state border it occurs, all within the United States.

The United States Code &ndash that is, the laws of the federal government, the United States of America &ndash contains scores of criminal offenses for which a sentence of death may be imposed upon conviction.

The modern federal death penalty dates from 1988. At first, it authorized capital punishment only for the leaders of large drug trafficking organizations “who intentionally kills or counsels, commands, induces, procures, or causes the intentional killing of an individual and such killing results.” Since then, Congress repeatedly has added to the offenses eligible for a death sentence, most notably in the Anti-Terrorism and Effective Death Penalty Act of 1996. Most federal death penalty eligible offenses involve either terrorism or the murder of a federal official.

Federal law mandates several unique procedures before imposition of a death sentence. For example, death alone must be imposed by a jury. Although a unanimous jury verdict is required for conviction of any crime in a federal court, normally the specific sentence is chosen by a judge acting alone. A judge may not impose a sentence of death, however, unless a jury unanimously recommends it.

Prior to recommending a death sentence, a federal jury must find that the defendant acted with one of four intents such as actual intent to kill or intentionally engaging in an act of violence with reckless disregard for human life.

If it finds one of those four mental elements, the jury continues on to consider aggravating and mitigating factors. In order for a defendant to be eligible for imposition of the death penalty, the government must establish beyond a reasonable doubt the existence of at least one statutory aggravating factor. Statutory aggravating factors vary with the offense charged, but they typically concern the nature of the defendant, the nature of the offense, and the nature of the victims. Due to the incendiary nature of evidence for aggravating factors such as whether “the defendant committed the offense in an especially heinous, cruel, or depraved manner in that it involved torture or serious physical abuse to the victim,” the jury hears such evidence in a special sentencing hearing only after having reached a unanimous guilty verdict. In addition, many federal judges bifurcate the sentencing hearing by requiring evidence and a special verdict on the mental element before evidence and deliberation on the aggravating and mitigating factors.

The United States Government has executed three people under the modern statute, but another 61 have been sentenced to death. Those 61 are in various stages of litigation challenging their conviction or sentence or both. This appellate process usually requires several years and sometimes even decades to complete.

An integral part of this federal system is the doctrine of dual sovereignty. As the United States Supreme Court explained in Bartkus v. Illinois, 359 U.S. 121, 138&ndash39 (1959):

Thus, the U.S. Constitution’s guarantee that “nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb” is interpreted to mean “twice by the same sovereign government.” Offenses punishable under both federal and state law may be prosecuted by both the federal and the state government, and sentences may be imposed by both courts to run consecutively to each other. Moreover, either can prosecute a defendant for violation of its own laws without regard to the outcome of the prosecution by the other sovereign government. Thus, a defendant convicted in a state court and sentenced to a term of imprisonment may be charged again for the same conduct and sentenced to an additional term of imprisonment in federal court. Or, a defendant acquitted in state court may be charged again for the same conduct and convicted in federal court. This usually is true regardless of which government goes first, although some state constitutions such as that of Pennsylvania prohibit subsequent prosecution after a prior prosecution in either state or federal court.

This doctrine of dual sovereignty can generate very controversial results in death penalty cases. States which have expressly considered and rejected the death penalty on grounds that it is immoral sometimes find their citizens prosecuted and sentenced to death in a federal court.

Consider the 2013 bombing of the Boston Marathon in the state of Massachusetts. Local prosecutors charged Dzhokhar Tsarnaev with murder and related charges under Massachusetts law. Massachusetts abolished the death penalty in 1984. Meanwhile, a United States Grand Jury has indicted Tsarnaev with federal offenses which do allow for a death penalty. Thus, Tsarnaev might be executed for his alleged crimes even though he is a resident of and committed them in a state which has banned capital punishment.

A state took an opposite approach in the case of Terry Nichols, who helped Timothy McVeigh destroy the Alfred P. Murrah Federal Building in downtown Oklahoma City in the state of Oklahoma, killing 168 people including 19 children and injuring several hundred more. The United States Government convicted and executed McVeigh. It also convicted Nichols, and the federal court sentenced Nichols to life in prison without the possibility of parole. Oklahoma officials, however, felt strongly that Nichols should die for his crimes. Unsatisfied with the federal sentence, they prosecuted Nichols and sought the death penalty under state law. The Oklahoma jury convicted Nichols, but it deadlocked on imposing the death penalty. The Oklahoma court then sentenced Nichols to 161 consecutive life terms without the possibility of parole.

In an attempt to make use of the death penalty consistent at least within the federal judicial system, federal law mandates that the Attorney General of the United States personally must decide whether to seek a death sentence in any federal case involving a statute which authorizes a death penalty. Thus, even if the federal prosecutor in the district where the prosecution occurs is opposed to seeking the death penalty, the Attorney General can and sometimes does order the prosecutor to seek it. Similarly, sometimes a federal prosecutor who wants to pursue a sentence of death in a particular case is prevented from doing so by the Attorney General. An extraordinary provision of federal law provides the defense an opportunity to make its case against seeking a death sentence to the Attorney General even before a notice of intent to seek the death penalty is filed in court. Once such a notice is filed, however, pursuit of a death sentence may not be withdrawn in return for a guilty plea by the defendant.

In short, the death penalty in the United States is as complex and diverse as the American federal system itself. The divergent positions of the Commonwealth of Massachusetts and of the United States of America in the closely watched case against Dzhokhar Tsarnaev may renew the debate about capital punishment.