WASHINGTON – The federal government will start carrying out death sentences for the first time in nearly two decades, Attorney General William Barr said Thursday, ordering officials to schedule executions for five inmates.

The U.S. Bureau of Prisons has not executed anyone since 2003, and it faced legal challenges to how it planned to carry out capital punishment. In reversing the informal moratorium, Barr ordered the government to adopt a new method for executing prisoners, replacing its lethal cocktail with injections of a single drug, pentobarbital.

“Congress has expressly authorized the death penalty through legislation adopted by the people’s representatives in both houses of Congress and signed by the President,” Barr said in a statement. “The Justice Department upholds the rule of law – and we owe it to the victims and their families to carry forward the sentence imposed by our justice system.”

The decision is a reversal by the federal government, which has executed three people since the death penalty was reinstated in 1988. President Donald Trump has frequently advocated for capital punishment.

The decision comes as the number of executions nationwide has declined in the past 20 years. Several states imposed moratoriums on lethal injections because of problems such as botched executions, in which prisoners gasped and groaned before dying, and the shortage of pharmacies willing to supply drugs that can kill swiftly and reliably without mishaps that might violate constitutional rights.

More:Death penalty sentences, executions remained at near-record lows in 2018

The federal government's new protocol is similar to the one used by states such as Georgia, Missouri and Texas, the Justice Department said. Fourteen states have used pentobarbital in more than 200 executions, and the Supreme Court upheld the use of the drug.

Anti-death-penalty activists decried the decision as an impingement on states' authority and a setback on human rights.

"As more countries, and U.S. states, recognize the cruel, inhumane, and arbitrary nature of capital punishment, this reversal is just another example of the United States' further downwards spiral backwards on human rights. The death penalty is a uniquely cruel and irreversible punishment. The U.S. should be putting this barbaric practice to an end, not ramping it up," Nicole Austin-Hillery, executive director of Human Rights Watch's U.S. program, said in statement.

It was unclear where the federal government would obtain pentobarbital, or whether it already has a supply of it. Lundbeck, a Danish pharmaceutical company that manufactured pentobarbital, has adamantly opposed the use of the drug in capital punishment. In 2011, the company denied distribution of the product to U.S. prisons that carry out executions and required purchasers to sign an agreement saying the drugs would not be made available for lethal injections.

Another company, Akorn, acquired the product and kept the restrictions in place.

The Justice Department and the Bureau of Prisons declined to comment on pentobarbital supplies.

"No one knows where the federal government bought its pentobarbital because it hasn't disclosed that information," said Megan McCracken, a lethal injection expert at the University of California-Berkeley School of Law. "The bottom line is that this is important information that needs to be disclosed."

In May, the Office of Legal Counsel issued an opinion saying the Food and Drug Administration does not have the authority to regulate drugs used for lethal injection, effectively making it easier for states and the federal government to carry out executions after pharmaceutical companies restricted supplies.

The new protocol, summarized in two pages, does not include a plan to go through a rulemaking procedure, as required by the Administrative Procedures Act. That means the new procedures don't have to be reviewed by the courts, McCracken said.

More:Supreme Court troubled by planned use of lethal injection to execute prisoner with rare condition

"There are ways in which a one-drug procedure is a step in the right direction because this procedure will not use paralytics or the potassium chloride, both of which add to the pain and suffering," McCracken said. But without knowing the qualifications of the team that will carry out the execution, "it is impossible to know whether this procedure will be constitutional."

The protocol keeps the identities of personnel, as well as documents about qualifications and training, private.

Robert Dunham, executive director of the Death Penalty Information Center, said the new protocol is an "end run" around judicial review.

"What you would hope is that if the government wants to be taking people's lives, that they would exercise caution and respect the law and the legal process as much as possible," Dunham said. "And it's really questionable that you would sign death warrants before you had a legally sanctioned protocol in place. ... The protocol needs to go through the court process, or the way in which the government carries out executions risks losing its legitimacy."

Barr on Thursday ordered officials to schedule the executions of five men, all of whom were convicted of murder and exhausted their appeals.

Daniel Lewis Lee, a member of a white supremacist group, was convicted of murdering a family, including an 8-year-old girl. Lezmond Mitchell was convicted of killing an elderly woman. Wesley Ira Purkey raped and murdered a 16-year-old girl. Alfred Bourgeois molested and beat to death his 2 1/2-year-old daughter. Dustin Lee Honken killed five people, including a mother and her two young daughters. Three of the men are scheduled to be executed in December and the other two in January 2020. All of the executions will take place at the federal penitentiary in Terre Haute, Indiana.

Scheduling the lethal injections within weeks of each other, in a facility where executions haven't been carried out in 16 years, is equally troubling, Dunham said.

"You increase the risk that something will go wrong by having the executions so close to one another," Dunham said. "You also magnify the amount of damage that you will inflict upon corrections personnel ... When they do take a life, it is highly traumatic."

It's unclear if the same or varying teams will carry out the executions.

There are 61 federal inmates on death row, according to a tally by the Death Penalty Information Center. Dunham said those inmates are very likely to challenge the new protocol. Two recent additions are convicted mass shooter Dylann Roof, who was sentenced in 2017, and Boston bomber Dzhokhar Tsarnaev, sentenced in 2015.

The last federal inmate executed was Louis Jones Jr., a disgraced Gulf War veteran who was killed in 2003 after he was convicted in the rape and murder of a 19-year-old Army recruit in Texas.Two others, Oklahoma City bomber Timothy McVeigh and drug kingpin Juan Raul Garza, were executed in 2001.

There have been nine executions nationwide this year, three of which were carried out by Texas and another three by Alabama. In 1999, nearly 100 prisoners were executed, according to the Death Penalty Information Center.

Contributing: Kevin Johnson and Richard Wolf