Members of the congregation react during a prayer service for Wednesday's shooting victims at the Morris Brown AME Church in Charleston, South Carolina June 18, 2015. [Photo/Agencies]

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

"I don't have any words for it," Cowles, 56, said in a telephone interview. "Nobody in my family had seen anything like this coming."

Roof was armed with a handgun but surrendered peacefully at his arrest, said Charleston Police Chief Greg Mullen.

In a Facebook profile apparently belonging to Roof, a portrait showed him wearing a jacket emblazoned with the flags of apartheid-era South Africa and of the former Rhodesia, now Zimbabwe, both formerly ruled by white minorities. Many of his Facebook friends were black.

Roof was arrested on two separate occasions at a shopping mall earlier this year for 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.

Sylvia Johnson, a cousin of Pinckney, told MSNBC that a survivor told her the gunman reloaded five times during the attack despite pleas for him to stop.

"He just said, 'I have to do it. You rape our women and you're taking over our country," Johnson said.

US Attorney General Loretta Lynch said her office was investigating whether to charge Roof with a hate crime motivated by racial or other prejudice.

Under federal and some state laws, such crimes typically carry harsher penalties, but South Carolina is one of just five US states not to have a hate-crimes law.