By Kristina Sgueglia

(CNN) — The busiest shopping day of the year also saw a major boom for gun sales, with the federal background check system expected to set a record of more than 144,000 background checks Friday, according to the FBI.

The staggering number of checks — an average of almost three per second, nearly three times the daily average — falls on the shoulders of 600 FBI and contract call center employees who will endure 17-hour workdays in an attempt to complete the background reviews in three business days, as required by law, FBI spokesman Stephen Fischer said.

“Traditionally, Black Friday is one of our busiest days for transaction volume,” Fischer said.

On average, more than 500 gun background checks a day fail because of incomplete information required for a decision, according to the FBI’s National Instant Criminal Background Check System, which is responsible for checks on firearm purchases from federally licensed shops.

Officials estimate that about 3,000, or 2%, of the more than 144,000 Black Friday background checks will not be completed because of insufficient information from records such as court documents.

Employees of the background check agency, who work every day but Christmas, will work through the weekend to vet Friday’s purchases.

As of 2 p.m. Friday, the agency had processed more than 94,000 transactions, on pace to surpass last year’s 144,758, Fischer said

“We are averaging three checks per second,” he said. “The challenge is to have staff keep up with this volume. We do that by limiting personal leave, asking employees to work extra shifts and reutilizing former … employees to serve in NICS during this busy period.”

The agency brings in 100 extra employees to deal with the increase.

“This means saving lives and protecting people from harm — by not letting guns fall into the wrong hands,” FBI Manager Kimberly Del Greco said in a statement. “It also ensures the timely transfer of firearms to eligible gun buyers.”

Overall, about 186,000 background checks a year cannot be completed, according to the FBI. It’s difficult to know exactly how many gun sales are authorized from that number because whether to make the sale is ultimately in the hands of the shop owner.

Last year, the agency completed 21 million background checks, and about 1.1% of those purchases were denied, the agency said.

Firearm background checks have doubled from the more than 9 million conducted when the system was implemented in 1999.

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

Gun purchasers are required to fill out a form from the Bureau of Alcohol, Tobacco, Firearms and Explosives with basic identification information and questions about the 10 disqualifying factors.

The gun shop can read the information to the background check agency over the phone or run the information through a secure Internet connection. The check sometimes involves calling courthouses to get records and dispositions.

“We won’t make a determination unless we are absolutely sure,” Fischer said.

However, the agency cannot deny a transaction based on an arrest without knowing the disposition of a case.

After the three business days have passed, completion of the sale becomes the prerogative of the licensed gun shop owner, according to the Brady Handgun Violence Prevention Act of 1998

Fischer said major retailers such as Walmart, Dick’s Sporting Goods, Cabela’s and Gander Mountain usually won’t go through with the sale without complete information.