BRIDGEPORT, W.Va. -- More gun sales than ever are slipping through the federal background check system - 186,000 last year, a rate of 512 gun sales a day, as states fail to consistently provide thorough, real-time updates on criminal and mental histories to the FBI.

At no time of year is this problem more urgent. This Friday opens the busiest season for gun purchases, when requests for background checks speed up to nearly two a second, testing the limits of the National Instant Criminal Background Check System, or NICS.

The stakes are high: In the U.S., there are already nine guns for every 10 people, and someone is killed with a firearm every 16 minutes. Mass shootings are happening every few weeks.

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"We have a perfect storm coming," FBI manager Kimberly Del Greco told The Associated Press during a rare glimpse into the inner workings of the system.

Much of the responsibility for preventing criminals and the mentally ill from buying guns is shouldered by about 500 men and women who run the system from inside the FBI's criminal justice center, a gray office building with concrete walls and mirrored windows just outside Bridgeport, West Virginia.

By federal law, NICS researchers must race against the clock: They have until the end of the third business day following an attempted firearm purchase to determine whether or not a buyer is eligible.

"They won't proceed or deny a transaction unless they are ABSOLUTELY certain the information they have is correct and sufficient to sustain that decision," FBI spokesman Stephen G. Fischer told the AP.

In roughly two percent of the checks handled by the FBI, agents don't get this information in time. If three business days pass without a federal response, buyers can legally get their guns, whether or not the check was completed.

Americans are buying more than twice as many guns a year now as they did when the background checks were first implemented in 1998. And that means more gun sales are effectively beating the system.

The federal government often takes the heat in debates over gun rights, but the FBI says states are largely to blame for this problem. They voluntarily submit records, which are often missing information about mental health rulings or criminal convictions, and aren't always rapidly updated to reflect restraining orders or other urgent reasons to deny a sale.

"We are stewards of the states' records," Del Greco said. "It's really critical that we have accurate information. Sometimes we just don't."

There are more than 48,000 gun retailers in the U.S., from Wal-Mart stores to local pawn shops. Store clerks can use the FBI's online E-Check System, which federal officials say is more efficient. But nearly half the checks are phoned in. Three call centers - in Kentucky, Texas, and Wheeling, W.Va. - take these calls from 8 a.m. to 1 a.m. every day but Christmas.

NICS did about 58,000 checks on a typical day last year. That surged to 145,000 on Black Friday 2013. They're bringing in 100 more workers than usual for the post-Thanksgiving rush this year.

The call centers have no access to privileged information about buyers' backgrounds, and make no decisions. They just type in their name, address, birthdate, Social Security Number and other information into the system. On Black Fridays, the work can be grueling: One woman took a call that lasted four hours when a dealer phoned in the maximum 99 checks.

"Rules had to be stretched," recalled Sam Demarco, her supervisor. "We can't transfer calls. Someone had to sit in her seat for her while she went to the bathroom."

In the years since these background checks were required, about 71 percent have found no red flags and produced instant approvals.

But ten factors can disqualify gun purchasers: a felony conviction, an arrest warrant, a documented drug problem or mental illness, undocumented immigration status, a dishonorable military discharge, a renunciation of U.S. citizenship, a restraining order, a history of domestic violence, or an indictment for any crime punishable by longer than one year of prison time.

Any sign that one of these factors could be in a buyer's background produces a red-flag, which sends the check to the FBI researchers to approve, deny or investigate. They scour state records in the federal database, and often call local authorities for more information.

"It takes a lot of effort ... for an examiner to go out and look at court reports, look at judges' documents, try to find a final disposition so we can get back to a gun dealer on whether they can sell that gun or not," Del Greco said. "And we don't always get back to them."

These workers have considerable responsibility, but little independent authority. They must use skill and judgment, balancing the rights of gun owners and the need to keep would-be killers from getting firearms.

Researcher Valerie Sargo said outstanding warrants often come up when they examine a red flag, and that can help police make arrests.

"It makes you feel good that this person is not supposed to have a firearm and you kept it out of their hands," she said.

It also weighs on them when the red flags aren't resolved in time. Tacked to a cubicle wall, a sign reads: "Our policy is to ALWAYS blame the computer."

FBI contractors and employees oversaw more than 9 million checks in the first full year, when the NICS system was established as part of Brady Handgun Violence Prevention Act of 1998. By last year, they oversaw more than 21 million. In all, only 1.25 percent of attempted purchases are denied. Denials can be appealed.

People can get guns without background checks in many states by buying weapons at gun shows or from individuals, a loophole the National Rifle Association does not want closed. But even the NRA agrees that the NICS system needs better data.

"Any database is only going to function as well as the information contained within," NRA spokesman Andrew Arulanandam said.

Del Greco doesn't see the states' data improving soon, which only adds to the immense challenge of getting through huge numbers of requisite checks on Black Friday.

A CBS News poll in December revealed Americans overwhelmingly support a federal law requiring background checks on all potential gun buyers, a measure that has so far failed to be put into law.