Earlier this year, he told an officer he wanted an AR-15 assault rifle.
A search of Roof's car turned up six crescent-shaped, 40-round magazines in the trunk.
"That's scary," former U.S. Attorney and State Law Enforcement Division Director Reggie Lloyd told The State
newspaper, which first reported the development.
Police had an eye on Roof for months before he shot dead nine African-American worshipers in cold blood on June 17. An officer arrested him on drug possession in late February.
During the bust, something bothered the officer. Roof had been asking suspicious questions at a shopping mall, The State reported: When does it close? And how many people work there?
The officer made note of the suspicious questions in a police report. Roof was released after the drug arrest on his own recognizance.
Then, on March 13, the same officer spotted Roof loitering in his car for hours at a public park. He was wearing all black, the police report said.
Because the officer was off-duty at the time, he alerted another to investigate. The second officer asked Roof if he could search the car. Roof complied. That's when the 40-round magazines turned up.
Roof also had a forearm for an AR-15 in his trunk. It allows a shooter to more easily keep a grip on the rifle, as the barrel grows fiery hot from the shots pouring out of it.
The church massacre
Police say Roof used .45 caliber handgun to carry out the church attack, stopping several times to reload.
An AR-15 fires more rounds per magazine and is more accurate at a longer range.
"This is frightening," State NAACP President Lonnie Randolph told The State. Roof was clearly thinking of killing more people, he said.
The white supremacist couldn't come up with the cash to get the big gun, he told an officer. He said he wanted it to take to a firing range, according to a Columbia police report.
On June 17, Roof toted the .45 Glock semi-automatic handgun it into a Bible study class at the Emanuel AME church in Charleston, where he sat with his victims through an hour's lesson before announcing he was there to kill black people.
Roof massacred nine, including Rev. Clementa Pinckney
, who will be buried on Friday.
President Obama will deliver Pinckney's eulogy
before mourners, who will include a bipartisan group of Washington lawmakers.
Hillary Clinton, House Speaker John Boehner and Vice President Joe Biden will attend the memorial service for Pinckney -- who was also a state senator -- giving it the feel of a state funeral.
His wanton killing triggered a tidal wave of sympathy for his victims and a national movement to remove from sight the Confederate battle flag
, which Roof revered as a symbol of white supremacy.
Many institutions and conservative politicians, who in the past have defended the flag flying have called for it to be pulled down. They include Gov. Nikki Haley Paul Thurmond, the son of dogged segregationist Strom Thurmond, and the Citadel military academy, whose cadets are credited for firing some of the first shots in the Civil War.
Another voice added
On Thursday, a prominent Southern Civil War enthusiast from South Carolina added his voice to the chorus. Glenn McConnell, president of the College of Charleston, has stirred controversy
over his participation in battle reenactments and photos of himself standing with African-Americans dressed as slaves.
"I support Governor Haley's call to remove the Confederate soldier's flag from State House grounds as a visible statement of courtesy and good will to all those who may be offended by it," he said in a statement.
Roof had wanted to kick off a race war. That appears to have backfired.