With just weeks left in office, US President Joe Biden has commuted the sentences of 37 of 40 federal death row inmates - potentially thwarting President-elect Donald Trump's plans to expand federal executions during his upcoming administration.

Biden's move was swiftly condemned by Republicans, with some accusing the president of siding with criminals over law-abiding Americans.

Federal executions were relatively rare before Trump's first term in office, which finished with a flurry of executions that ended a 130-year-old precedent of pausing executions amid a presidential transition.

He has vowed to resume the practice when he returns to the White House in January, setting the stage for possible legal battles early in the administration.

Here's what we know.

On Monday, Biden commuted the sentences of 37 out of 40 death row inmates, switching their penalty to life without parole.

Only three inmates were left to face the death penalty, including convicted Boston marathon bomber Dzhokhar Tsarnaev and Robert Bowers, who was sentenced to death for killing 11 worshippers and wounding seven during a shooting at a the Tree of Life synagogue in Pittsburgh in 2018.

The third, Dylann Roof, was sentenced to death in 2017 for a mass shooting that left nine black parishioners dead at the Mother Emanuel AME church in Charleston, South Carolina, in 2015.

While the move was widely praised by human rights groups such as Amnesty International, it was quickly condemned by some Republicans, as well as Trump's transition team and political allies.

In a statement, Trump communications director Steven Cheung said that "these are among the worst killers in the world and this abhorrent decision by Joe Biden is a slap in the face to the victims, their families, and their loved ones.

"President Trump stands for the rule of law, which will return when he is back in the White House," he added. Trump cannot undo the commutations when he returns to the White House next month.

Texas Republican Chip Roy posted on X that the decision was "unconscionable" and an abuse of power "to carry out a miscarriage of justice".

Another Republican, Arkansas Senator Tom Cotton, said that "when given the choice between law-abiding Americans or criminals, Joe Biden and the Democrats choose criminals every time."

Some family members also expressed anger.

On Facebook, Heather Turner - whose mother was killed in a 2017 bank robbery - called the commutations a "gross abuse of power".

"At no point did the president consider the victims," she wrote. "He, and his supporters, have blood on their hands."

The commutations do not apply to the approximately 2,200 death row inmates convicted by state courts, over which the president holds no authority.

Over the course of his election campaign, Trump vowed to resume federal executions and make more people eligible to receive the death penalty, including those convicted of raping children or drug and human-trafficking cases, as well as migrants who kill US citizens or police officers.

"These are terrible, terrible, horrible people who are responsible for death, carnage and crime all over the country," Trump said when he announced his presidential candidacy in 2022.

"We're going to be asking everyone who sells drugs, gets caught, to receive the death penalty for their heinous acts," he added.

There are more than 40 federal laws that can, in theory, result in the death penalty, ranging from murders committed during a drug-related shooting to genocide.

Almost all - with the exception of espionage and treason - explicitly involve the death of a victim.

Trump, however, has provided few details on how he plans to accomplish his campaign pledge.

Despite the lack of clarity, Trump's vows to expand the federal death penalty have elicited strong warnings from human rights advocates.

In an 11 December statement, for example, the American Civil Liberties Union said Trump's "chilling" plans amount to an expansion of the "killing spree he initiated in the final six months of his first presidency".

"He's already shown us that he will act on these promises," the statement said.

The inmates executed during the waning days of Trump's first administration included Lisa Montgomery, the first woman executed by the federal government since 1953, and Lezmond Mitchell, the only Native American on federal death row.

US media reported that Trump cannot reverse Biden's commutations.

Trump's efforts to expand the death penalty to crimes that do not involve murder are likely to face legal challenges.

In 2008, for example, the Supreme Court ruled that those convicted of raping children cannot be executed, adding that it's unclear if the death penalty could be applied to crimes in which a victim is not killed.

According to the National Registry of Exonerations, cases with child victims are particularly prone to wrongful convictions, can be "extremely emotional" and pit family members against one another.

Any further expansion of crimes that are eligible for the federal death penalty would require Congress to act and change the law.

In 2024, two bills - both sponsored by Florida Republican and Trump ally Anna Paulina Luna - sought to expand the use of capital offences to include possession of child pornography, as well as the trafficking, exploitation and abuse of children.

Both failed to pass in the House of Representatives.

Trump is also unlikely to be able to quickly re-populate the pool of federal death row inmates, as most death penalty cases take years and are subject to lengthy appeals processes.

While he does not have any direct authority over state executions, some experts have warned that Trump's pro-death penalty stance may trigger more executions at a state level.

"His rhetoric can and has spurred draconian measures and attitudes by leaders in states on several issues, including in the context of the criminal legal system," Yasmin Cader, a deputy legal director at the American Civil Liberties Union and director of the Trone Center for Justice and Equality told CNN.

In addition to the federal government and US military, 27 US states still have the death penalty on the books.

A Gallup poll conducted in October found that a slim majority of Americans - 53% - support the death penalty for convicted murderers, up from 50% a year before.