Acting on President Donald Trump's call for tougher penalties on violent crimes, Barr directed the Federal Bureau of Prisons to adopt a new lethal injection protocol to clear the way to carry out death sentences.
"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," Barr said in a statement.
There were 25 executions in the US last year, all carried out by state authorities on people convicted on state charges.
But debate about the methods of execution and controversy over the drugs used, as well as reticence from Trump's predecessor Barack Obama, means that no federal prisoner has been put to death since 2003.
Barr ordered the Bureau of Prisons to carry out executions using a single lethal injection of the barbiturate pentobarbital, replacing the previous, three-drug cocktails using thiopental.
"Since 2010, 14 states have used pentobarbital in over 200 executions, and federal courts, including the Supreme Court, have repeatedly upheld the use of pentobarbital in executions as consistent with the Eighth Amendment" of the Constitution, which bars cruel and unusual punishment, the Justice Department said.
SC overturned 78% death penalty verdicts in 8 years
There are 62 federal death row prisoners in the US, according to the Death Penalty Information Center, including Dzhokhar Tsarnaev, who was convicted of the 2013 Boston Marathon bombing that left three people dead.
The list also includes white supremacist Dylann Roof, who murdered nine African Americans in a Charleston, South Carolina church in 2015.
On Barr's order, the Federal Bureau of Prisons has scheduled executions for five people all convicted 15 years ago or more in brutal murders that involved children.
They include Daniel Lewis Lee, who robbed and killed a family of three including an eight-year-old girl in 1996, and Alfred Bourgeois who tortured and sexually molested his two-year-old daughter before killing her in 2002.
Federal executions were on hold for nearly four decades until 2001, when Oklahoma City bomber Timothy McVeigh was executed.
Two more people were put to death in federal prisons over the subsequent two years, and then the federal executions were halted again.
Since then all executions have been carried out by states. Twenty-five of the 50 US states maintain an active death penalty, while 21 do not allow it and four have suspended its use.
Obama, president from 2009 to 2017, called the practice "deeply troubling," pointing to the disproportionately high number of African Americans sentenced to death, and some recent "rather gruesome and clumsy" executions by states.
The Justice Department has pressed to toughen punishments for violent crimes since Trump took office, but Barr's move also comes as the president seeks to bolster his law-and-order credentials ahead of next year's election.
In October 2017 Trump called for the execution of Sayfullo Saipov, the Uzbek immigrant accused of a truck attack on pedestrians in New York in October 2017 that killed eight people.
He repeated that call after a white nationalist killed 11 in October 2018 in a Pittsburgh synagogue.
Both of those cases remain in the courts.
Death penalty opponents blasted the policy change and called for a court-ordered delay in the executions, the first of which is scheduled for early December.
"The federal death penalty is arbitrary, racially-biased, and rife with poor lawyering and junk science," said Ruth Friedman, Director of the Federal Capital Habeas Project.
"These and other concerns, including troubling questions about the new execution protocol, are why there must be additional court review before the federal government can proceed with any execution."
Morris Moon, an attorney for Lee, whose execution is scheduled for December 9, said his case was based on unreliable evidence, and that Lee's accomplice in the crime killed the child and yet was sentenced to life in prison.
"The trial judge, the lead prosecutor, and members of the victims' family all oppose executing him and believe a life sentence is appropriate," Moon said.
"Given the problems that undermine the fairness and reliability of Danny Lee's conviction and death sentence, the government should not move forward with his execution."
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