Black Friday is traditionally one of the busiest days for gun sales in the U.S. Pictured, Metro Shooting Supplies' employee Chris Cox, left, speaks to a customer about the purchase of a 9mm handgun in Bridgeton, Missouri, Nov. 13, 2014. Reuters

The Federal Bureau of Investigation is working overtime to process the more than 144,000 background checks it is expected to receive on Black Friday, traditionally one of the busiest days for gun sales in the U.S. This year, the bureau anticipates a record number of requests for checks, according to CNN.

"We are averaging three checks per second," FBI spokesman Stephen Fischer told CNN. "The challenge is to have staff keep up with this volume.” He said employees are asked to work extra shifts and that the agency limits personal leave. The FBI department in charge of firearms background checks, the National Instant Criminal Background Check System, has a staff of about 600. Many of them will put in 17-hour workdays this weekend to keep up with the increased demand for guns on Black Friday.

The sudden surge in checks puts a strain on the agency, which has only until the end of the third business day after someone has attempted to purchase a firearm to determine the buyer’s eligibility. If the agency has not completed the background check by the time the clock runs out, the gun buyer has the right to get his or her gun anyway, at the discretion of the vendor, the Associated Press reported.

Last year, more than 186,000 background checks were not completed on time, according to the AP, although it is unclear how many of those resulted in customers leaving with new firearms. The agency typically processes about 58,000 checks a day. About 500 of them fail; a similar number slip through the cracks daily before the FBI can finish processing.

The reason the FBI cannot always process checks on time is that states often fail to provide the agency with up-to-date criminal and mental histories, according to ABC News. For a day like Black Friday when more people try to buy guns than any other day of the year, the stakes are even higher. Officials estimate that this year, about 3,000 background checks submitted on Black Friday will not be completed because of missing information, CNN noted.

A person can be prohibited from purchasing a gun for one of several reasons, including a felony conviction, a domestic abuse record, or certain mental health histories.