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The FBI said Friday that the man accused of killing nine African American parishioners at a historically black church in Charleston should not have been able to purchase a gun.

Officials said a background check failure allowed Dylann Roof to illegally purchase a .45-caliber Glock handgun on April 11, eight days after he turned 21, at Shooter's Choice in West Columbia, South Carolina. Roof had previously acknowledged drug possession, according to the FBI.

Law enforcement officials had previously said the transaction was entirely legal, despite his pending drug charge. The weapon was purchased at the gun store, which is 25 miles from his home.

FBI director James B. Comey called that assessment "a mistake in a matter of heartbreaking importance."

“We are all sick this happened,” he said. “We wish we could turn back time.”

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Related: Charleston Church Gunman Dylann Roof Bought Pistol Locally: Officials

Comey said there were a number of missteps that led to Roof being able to purchase the gun.

Roof had been arrested in late February at a Columbia shopping mall by Columbia police in Lexington County and he admitted possessing Suboxone, a controlled substance commonly used to treat heroin addiction. He was indicted by a Lexington County grand jury on a state drug charge.

If a National Instant Criminal Background Check System examiner had seen that, during his arrest, Roof admitted possessing drugs, his gun purchase would have been denied, Comey said.

"But the examiner never saw that," Comey said.

Instead, she followed protocol after learning Roof had been arrested in South Carolina earlier that year on a felony drug charge and checked with several local jurisdictions over the next three days to verify information.

Roof's rap sheet listed the Lexington County Sheriff's Office as the arresting agency, rather than the Columbia Police Department. As the examiner sought to suss through the confusion about which jurisdiction had Roof's case, the mandatory three day waiting period to clear or deny a purchase expired and Roof was sold the weapon he ultimately used in the deadly church shooting.

Those on both sides of the gun policy debate have long highlighted problems with the National Instant Criminal Background Check System. Loopholes have been discovered before in the background check system, also known as NICS, which the Obama administration once described as its "most important tool" for stopping gun crime.

Comey directed officials Friday to meet with the families of those killed in the shooting massacre during a bible study at Emanuel African Methodist Episcopal Church to explain the breakdown that enabled Roof's gun purchase.

"Before I wasn’t angry, now I’m angry," said Rev. Anthony Thompson, whose wife, Myra, was one of the parishioners killed.