The Federal Bureau of Investigation (FBI) said South Carolina mass shooting suspect Dylann Roof "should not have been allowed to purchase the handgun" last April but a lapse in background check enabled him to do so.

FBI Director James Comey said a National Instant Criminal Background Check System (NICS) examiner failed to see Roof's arrest record in which he admitted possessing drug that should have automatically barred him from purchasing the gun that he allegedly used to kill nine people in the Emanuel African Methodist Episcopal Church in Charleston, South Carolina, last June 17.

Roof, Comey said, tried to buy a gun from a store in West Columbia, South Carolina, on April 11, a Saturday. On April 13, a federal examiner at the FBI's Criminal Justice Information Services in West Virginia began processing his gun application.

All firearm sellers in the US are required to submit information to the NICS, which has three business days to perform a background check and clear or deny the purchase. If no decision has been given after three days, the sellers have the discretion to sell the gun.

Roof's background check showed that he was arrested last March in South Carolina on a felony drug charge.

"This charge alone is not enough to deny proceeding with the transaction. As a result, this charge required further inquiry of two potential reasons to deny the transaction. First, the person could have been convicted of a felony since the arrest. Second, the underlying facts of the arrest could show the person to be an unlawful drug user or addict," Comey explained.

The NICS examiner saw Roof's records from the Lexington County court showing that he was still a defendant in a case but not yet convicted.

However, the Lexinton County police referred the NICS examiner to the Columbia Police Department in South Carolina.

The examiner failed to find the contact information of the Columbia police and instead contacted the West Columbia police, which said they had no record of Roof.

When the NICS did not respond to the gun store's query by April 16, the dealer allowed Roof to purchase the gun.

After the Charleston shooting, the FBI found the "rap sheet confusion—listing the arresting agency as the Lexington County Sheriff—and the internal contact sheet omissions were discovered."

Comey has ordered a review of the case and gave the FBI's Inspection Division 30 days to complete it.

"We are all sick that this has happened. We wish we could turn back time, because from this vantage point everything seems obvious, but we can't," he said.

However, pro-gun organisations said background checks are a failure.

Guns of America's Erich Pratt said, "The entire background check is flawed. Not only is it unconstitutional, it's already failing to keep guns out of the criminals' hands."

"Arguing that we can make background checks better to stop criminals from getting guns is the very definition of insanity," Pratt said.