WASHINGTON » The FBI’s surprise admission Friday that a bungled background check allowed Dylann Roof to buy the .45-caliber pistol he is accused of using to kill nine people at a Charleston, S.C., church set off calls for tougher firearm screening rules and may give the White House a chance to reopen a push for gun-control legislation.

“I’m here to talk to you about a mistake in a matter of heartbreaking importance to all of us,” FBI Director James B. Comey said at a news briefing. “Dylann Roof, the alleged killer of so many innocent people at the Emanuel AME church, should not have been allowed to purchase the gun he allegedly used that evening.”

Those words immediately set off demands from congressional Democrats to lengthen the federally mandated three-day waiting period used to conduct background checks into potential gun purchasers.

And it just as quickly triggered complaints by gun-rights’ advocates that millions of law-abiding gun owners should not pay for the mistake of an FBI background examiner who missed that Roof had recently admitted that he possessed illegal drugs. Had the examiner realized that, she would have immediately disqualified him from purchasing the weapon that authorities say was used two months later in the attack on an evening Bible study group at the historic African-American church.

Comey, who dispatched FBI officials to Charleston on Friday to discuss the mistake with the families of those killed and wounded in the June 17 attack, announced he was conducting a 30-day inquiry into why the error occurred and how future lapses can be avoided.

In addition, the Department of Justice’s inspector general’s office is examining the three-day waiting period, which was at the heart of how the gun made its way legally to Roof, an avowed white supremacist.

“What we can do is make sure that we learn from it, get better, and work to ensure that we catch everything,” Comey said in announcing the process review.

In explaining the error, Comey said Roof attempted to purchase the weapon at a gun shop in West Columbia, S.C, on Saturday, April 11. On the next business day, Monday, April 13, an FBI examiner in the bureau’s West Virginia facility was assigned the case. Under the law, if the bureau does not disqualify a buyer within three business days, a gun sale is allowed to proceed.

Her initial check showed Roof had been arrested in nearby Columbia on March 1 on a felony drug charge. But because he had not been convicted, there were no automatic grounds to block the sale.

Seeking additional details about the arrest, she faxed a request to the Richland County sheriff’s office. But it turned out that the arrest was made in a part of Columbia that is located in nearby Lexington County, so Richland County officials responded that they had no record of Roof’s arrest.

She next faxed the Lexington County prosecutor’s office for information, but did not hear back, Comey said. Had they responded, he said, prosecutors probably would have shared with the examiner a police report in which Roof admitted he had “possessed” illegal drugs. Under federal law, admitting drug possession is enough to stop a gun sale.

Working on 15 to 20 other cases at the time, the examiner placed the Roof file in her “delayed/pending” queue. On Thursday, April 16, when the three days had passed, Roof was cleared to buy the gun.

At the White House, spokesman Josh Earnest suggested the FBI’s technical lapse could lead to reforms. “There are some common-sense steps that can be taken to reduce gun violence,” he said, “and there are some steps that Congress could take right now that have the strong support of the American people — that even have the strong support of gun owners in this country.”

He added that those steps “could have the positive impact of keeping guns out of the hands of criminals or others who shouldn’t be able to get their hands on them.”

In the Senate, the ranking Democrat on the Judiciary Committee, Sen. Patrick J. Leahy of Vermont, said, “We simply cannot have such failures in our background check system, and peoples’ lives are at stake. Clearly, more oversight is needed.”

In the House, Rep. Carolyn B. Maloney, a New York Democrat and leading advocate for more gun control, called for hearings by the Oversight and Government Reform Committee. “In the face of this overwhelming and tragic evidence of dangerous loopholes … Congress must step forward to examine its record and evaluate proposals to improve these background checks,” she said.

But Republican leaders who control both sides of Congress called for restraint. Sen. Charles E. Grassley of Iowa, chairman of the Judiciary Committee, said the problem is not with the background check process but how it is administered. That, he said, would seem to “undercut” any attempts by Democrats to enact “unnecessary gun laws.”

Larry Pratt, executive director of the firearms lobbying organization Gun Owners of America, reacted angrily to the idea of politicians using the actions of a “dirtbag” to lengthen the purchase review process from three days to a week or more. “And if they can get seven, then they’ll go for 14,” he said.

The National Rifle Association said it was reviewing Comey’s statements and the political reaction, and would respond soon.

Comey, who learned of the problem Thursday, defended the examiner as someone with experience — “a veteran.” He added that the miscue “breaks all of our hearts. It’s very painful.”

In Charleston, J. Elliott Summey, chairman of the Charleston County Council, was hesitant to blame the FBI or local police, but said the lapse highlighted the failure of gun laws.

“Either way, I think that’s a very sad scenario: that we’ve got gun laws to try to help legislate responsibility, and somehow it’s not foolproof.”

He added, “How do you tell those nine families they missed it? … It’s not going to bring those people back.”

Times staff writer James Queally in Los Angeles contributed to this report.