Lawyers are arguing that the death penalty is unconstitutional in the case of a white man who orchestrated a shooting spree at a historically black South Carolina church.
Dylann Roof is charged with murdering nine parishioners during a Bible study session at the Emanuel AME Church, Charleston, in July 2015. He was 21 years old at the time.
Roof faces federal hate crime charges in connection with the killings too - he had appeared in photos waving Confederate flags and burning or desecrating US flags. During the attack he hurled racial abuse, survivors told police.
But a motion has been filed challenging the federal government's intention to seek the death penalty in his murder trial, calling it a "cruel and unusual punishment".
"[T]his Court should rule that the federal death penalty constitutes a legally prohibited, arbitrary, cruel and unusual punishment prohibited by both the Fifth and Eighth Amendments," the lawyers write in the filing.
Roof's lawyers also argue that the death penalty itself is unconstitutional, as is the federal death penalty law.
The federal government rejected Roof's offer to plead guilty and accept multiple life sentences without the possibility of parole.
Roof's lawyers are also challenging "death qualification" - this is the jury selection process where a jury willing to impose the death penalty is found.
They write that this process has "no constitutional or statutory underpinnings" and that there is no justification for maintaining it because it "increases the influence of racism and sexism on the death determination".
There are other cases where the federal death penalty is under challenge - take the case of Donald Fell in Vermont recently, where a federal judge finished a nine-day hearing over issues raised by his lawyers about the constitutionality of the death penalty.
Though the justice department says it is committed to seeking the death penalty, federal executions are actually exceedingly rare.