Attorney General Merrick Garland ordered a temporary halt to federal executions as the Department of Justice launches a review of death penalty procedures.

       In a memo obtained by NBC News, Mr Garland said there were serious concerns about the “troubling number of exonerations” in cases where prisoners are sentenced to death.

       "The Department of Justice must ensure that everyone in the federal criminal justice system is not only afforded the rights guaranteed by the Constitution and laws of the United States but is also treated fairly and humanly," he wrote to senior officials.

       Mr Garland reportedly ordered a review of a Trump-era procedure to carry out lethal injections using a single drug after the previous method using three drugs led to a pause in executions for almost 20 years due to a shortage of one of the drugs.

       After federal prison officials were directed during the previous presidency in 2019 to use the single-dose injections, 13 people were executed.

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       While eliminating the federal death penalty was a major part of Joe Biden’s presidential campaign, the Justice Department under a Biden White House sought to reinstate the death penalty for Boston Marathon bomber Dzhokhar Tsarnaev.

       When asked about the conflict in March, spokeswoman Jen Psaki said the president made clear during the campaign that he had “grave concerns whether capital punishment, as currently implemented, is consistent with the values that are fundamental to our sense of justice and fairness”.

       “The President also believes that the Department of Justice is an independent institution. And, of course, there are times where it is appropriate to have conversations, but I don’t have any conversations to preview for you with the Attorney General,” she told reporters during a daily briefing.

       The Justice Department’s 14 June brief to the US Supreme Court asking for the death penalty to be reinstated for Tsarnaev would likely not be affected by Mr Garland’s memo, as it does not outline whether the federal government would continue to seek the death penalty.

       The review also ordered the department to study the regulation used under the Trump administration that allowed federal prisons to execute prisoners with any method authorized by the state it is conducted.

       As the memo is directed at federal executions for inmates convicted of federal crimes, states that also have the ability to sentence offenders to death will likely not be affected by the temporary pause.