Authorities on Thursday arrested a 21-year-old white man suspected of killing nine people at a historic African-American church in Charleston, South Carolina in a rampage that the United States is investigating as a hate crime.

Law enforcement officials caught alleged gunman Dylann Roof, whose assault on Wednesday came in a year that has seen months of racially charged protests across the United States over killings of black men.

Roof was arrested after a traffic stop in Shelby, North Carolina, about 220 miles (354 km) north of Charleston, said police chief Gregory Mullen.

"This individual committed a tragic, heinous crime last night," Mullen told reporters.

U.S. Attorney General Loretta Lynch said her office was investigating whether to charge Roof with a hate crime motivated by racial or other prejudice. Such crimes typically carry harsher penalties.

The Southern Poverty Law Center, which researches U.S. hate groups, said the attack illustrates the dangers that home-grown extremists pose.

"Since 9/11, our country has been fixated on the threat of Jihadi terrorism. But the horrific tragedy at the Emanuel AME reminds us that the threat of homegrown domestic terrorism is very real," the group said in a statement, referring to the Sept. 11, 2001 attacks on the United States.

A man who identified himself as Roof's uncle earlier told Reuters Roof's father had recently given him a .45-caliber handgun as a birthday present and that he had seemed adrift.

"I actually talked to him on the phone briefly for just a few moments and he was saying, 'Well, I'm outside practicing with my new gun'," the uncle, Carson Cowles, 56, said in a telephone interview.

The victims, six females and three males, included Reverend Clementa Pinckney, who was the church's pastor and a Democratic member of the state Senate, according to colleagues.

Roof sat with churchgoers inside Emanuel African Methodist Episcopal Church for about an hour on Wednesday before opening fire, Mullen said.

Demonstrations have rocked New York, Baltimore, Ferguson, Missouri and other cities following police killings of unarmed black men including Eric Garner, Freddie Gray and Michael Brown.

A white police officer was charged with murder after he shot Walter Scott, an unarmed black man, in April in neighboring North Charleston.

'A LOT OF CONCERN'

The local community reacted with shock and grief after Wednesday's mass shooting.

"This is going to put a lot of concern to every black church when guys have to worry about getting shot in the church," said Tamika Brown at an AME church near the site of the shooting while waiting for a noon prayer vigil. "They might need security guards, police officers."

Eight victims were found dead in the church, Mullen said, and a ninth died after being taken to hospital. Three people survived the attack. Officials did not immediately identify the other victims.

Roof was charged on two separate occasions earlier this year with a drug offense and trespassing, according to court documents.

Roof's mother, Amy, declined to comment when reached by phone.

"We will be doing no interviews, ever," she said before hanging up.

The shooter told one survivor he would let her live so she could tell others what happened, the president of the Charleston NAACP, Dot Scott, told the local Post and Courier newspaper.

Roof reloaded five times despite pleas for him to stop shooting during a Bible-study group, a cousin of Pinckney said.

"It is a very, very sad day in South Carolina, but it is a day that we will get through," Governor Nikki Haley, a Republican, told reporters.