But that doesn't mean Roof will be executed any time soon -- if ever.
But a spate of factors could delay or even prevent his federal execution -- already an extremely rare event.
Very rarely -- especially compared with some of the 31 states that enforce capital punishment.
In fact, the federal government hasn't executed anyone in 14 years.
The federal death penalty was reinstated in 1988 after a 16-year moratorium. Since then, only three federal inmates have been executed -- all in Terre Haute, Indiana.
-- Timothy McVeigh
, for the 1995 Oklahoma City bombing that killed 168 people. He was executed on June 11, 2001.
-- Juan Raul Garza
, who was convicted of killing three people and running a marijuana drug ring in Texas. He was executed on June 19, 2001.
-- Louis Jones, for the kidnapping and murder of 19-year-old Army Pvt. Tracie McBride. He was executed on March 18, 2003.
By contrast, South Carolina has executed 41 people since 1988
Why are federal executions rare compared with the states?
Because the federal death penalty didn't exist for much of the 1970s and '80s, there have been far fewer federal death penalty cases tried in recent decades than at the state level.
And since the appellate process can take decades, that helps explain the dearth of recent federal executions.
"There aren't nearly as many (federal) cases that have moved through the complete appellate process," said Robert Dunham, executive director of the Death Penalty Information Center.
What about Roof's state charges?
Roof also faces 13 charges in South Carolina, including nine counts of murder, three counts of attempted murder and one count of possession of a handgun during the commission of a violent crime.
But the state trial, originally scheduled to start next Tuesday, has been delayed indefinitely, CNN affiliate WCSC
reported. The apparent reason: to provide more time to prepare the state case following the close of the federal one.
Where will Roof be held?
Roof is now the 63rd person on federal death row.
Had he not been facing state charges, Roof would go to the federal death row facility in Terre Haute, Indiana.
But with his state trial still looming, he will stay in South Carolina for the foreseeable future, Dunham said.
Could Roof get sentenced to death in state court, too?
Yes. In fact, prosecutors have said they'll seek the death penalty at the state level.
If the state condemns him, who would execute Roof?
If Roof also gets sentenced to death in state court, the answer depends on which side finishes the appeals process first.
"When people have been sentenced to death in more than one jurisdiction, the appellate process proceeds in both," Dunham said.
"Whichever one finishes the process first has the ability to issue a death warrant and to carry out the execution."
How long could appeals take?
It's not uncommon for death penalty appeals to take years, if not decades.
Roof's case is unusual because he represented himself for part of his federal trial. And that could complicate his appeals, Dunham said.
"Somewhere in the appeals process, someone will probably raise the question of whether it was appropriate to allow him to represent himself," he said.
Roof actually could kick off his federal appeal by petitioning for a second federal sentencing trial, former South Carolina prosecutor Holman Gossett told CNN affiliate WSPA
"He didn't have any attorneys helping him in the penalty phase, so he may make that motion after reflecting on it," Gossett said. "Then it would go through the process of automatic hearings with appellate courts to see if there's any reason under the law that it should not stand legal grounds."
How might Roof avoid execution?
Two factors are most likely to stand in the way of his sentence.
"The most likely outcome once the death penalty is imposed is it's overturned in the courts," Dunham said.
Reasons can include juror misconduct, improper jury instructions or poor representation by defense counsel, he said.
Then there's the issue of lethal injection drugs -- or lack thereof.
"There's no FDA-regulated pharmaceutical company in the United States that's willing to sell its products for use in execution," Dunham said.
That's a problem for the feds and South Carolina officials.
Right now, the state's Department of Corrections does not have the drugs necessary to carry out executions, spokeswoman Sommer Sharpe said.
"South Carolina, along with many other states, are making efforts to obtain lethal injection drugs," she said.
Lethal injection initially required a three-drug cocktail: The first (sodium thiopental or pentobarbital) puts the prisoner to sleep; the second (pancuronium bromide) brings on paralysis; and the final agent (potassium chloride) stops the heart.
In 2010, European drug manufacturers began to ban exports of the cocktail ingredients to the United States.
The next year, concerned about the use of sodium thiopental in executions, Illinois-based Hospira stopped making the drug, and Denmark-based Lundbeck banned U.S. prisons from using its pentobarbital.
The United Kingdom also introduced a ban on exporting sodium thiopental, and the European Union took an official stance in 2012 with its Regulation on Products used for Capital Punishment and Torture.
Do victims' relatives want the death penalty?
Melvin Graham, whose sister Cynthia Hurd was killed by Roof, supported the jury's decision to recommend the death penalty.
"Today we had justice for my sister," Graham said Tuesday.
"This is a very hollow victory, because my sister is still gone," he said. "I wish that this verdict could have brought her back. But what it can do is just send a message to those who feel the way he feels that this community will not tolerate it."
In one case, the children of Kelly Gissendaner -- a woman convicted of helping kill her husband -- said they had made peace with their mother and begged the state of Georgia to not kill their one remaining parent
But the children's appeals failed, and Gissendaner was executed in 2015.