(CNN) - The nation's gun buying background check system is supposed to keep firearms out of reach for dangerous individuals. But it's not meant to catch early warning signs like those exhibited by Nikolas Cruz, the man identified as the shooter who allegedly killed 17 at a South Florida high school on Valentine's Day.
On Thursday, details emerged about Cruz's extensive presence online. He brandished guns and knives while making racist remarks against Muslims. And, it appears, he even made explicit threats that foreshadowed Wednesday's massacre.
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Last fall, a YouTube user going by the name "nikolas cruz" left the following comment beneath another person's video: "Im going to be a professional school shooter."
However, Cruz passed the FBI's background check to be able to purchase his AR-15-style semiautomatic rifle in the past year, according to a U.S. official briefed on the investigation.
He was able to pass the background check because the system screens for certain indicators of past violence, misconduct, and mental health issues --- but not all. The scope of review is narrow and would not capture all aspects of Cruz's profile, even if it included explicit, public threats against students and school disciplinary behavior that led to his expulsion from high school.
How federal gun licensing works
The Bureau of Alcohol, Tobacco, Firearms and Explosives is the agency that licenses gun dealers.
The ATF defines dealers as people who "repetitively buy and sell firearms with the principal motive of making a profit." The ATF says that people who "make occasional sales of firearms from your personal collection" do not need to be licensed.
The agency focuses on whether the seller presents him- or herself as a dealer. For example, if the seller is advertising, has a business card, or accepts credit cards, the ATF would see those as signs of a professional dealer. Restocking inventory is also a sign.
The ATF says all dealers must be licensed, regardless of whether they're selling guns in cyberspace, at gun shows, or at brick-and-mortar stores.
In some states, unlicensed sellers can make private sales without conducting a background check.
How background checks are conducted
All federally licensed gun dealers must run checks on every buyer, whether a purchase is made in a store or at a gun show.
The checks work like this: A buyer presents his or her ID to the seller and fills out ATF Form 4473 with personal information such as age, address, race, and criminal history, if any.
The seller then submits the information to the FBI via a toll-free phone line or over the internet, and the agency checks the applicant's info against databases. The process can take as little as a few minutes.
Required information includes name, address, place of birth, race, and citizenship. A Social Security number is optional, though it's recommended. The form also asks questions such as:
-- Have you ever been convicted of a felony?
-- Have you ever been convicted of a misdemeanor crime of domestic violence?
-- Are you an unlawful user of, or addicted to, marijuana or any other depressant, stimulant, narcotic drug, or any other controlled substance?
-- Are you a fugitive from justice?
-- Have you ever been committed to a mental institution?
Of course, many guns are bought and sold illegally; others are sold legally, but without any background check, since the system is only used by gun sellers with a federal license.
The federal government does not track nationwide gun sales, so reliable data on how many are sold is scarce.
Background checks: By the numbers
The FBI processed 25,235,215 background checks in 2017.
That was down from a record year in 2016 that saw more than 27,500,000 background checks.
The FBI's National Instant Criminal Background Check System, also known as NICS, operates seven days a week, 17 hours a day, out of a facility in Clarksburg, West Virginia. It is open on holidays, except Christmas.
Since the NICS system was started 20 years ago, the FBI has conducted 280,482,910 background checks. The FBI counts 1,504,808 "federal denials" over the years, most due to the individual's criminal history. Other reasons include "adjudicated mental health" (33,569 denials) and "illegal/unlawful alien" (23,279 denials). The FBI says 93 applicants were denied because they had renounced their U.S. citizenship.
A short history of background checks
Gun background checks were first mandated by the Brady Handgun Violence Prevention Act of 1993. The law was named after Jim Brady, the press secretary to President Reagan who was left paralyzed by John Hinckley's attempted assassination of Reagan in 1981. Brady died in 2014.
The Brady Campaign, a leading gun control lobby, says background checks work but wants Congress to do more and close exceptions to federal checks. At gun shows right now, for instance, generally only federally licensed dealers must do checks.
Meanwhile, the Brady Campaign and other groups have helped spur gun laws at the state level.
The last major attempt at federal gun control was about five years ago. President Barack Obama and Democratic leaders proposed a plan to expand background checks and restrict some semi-automatic weapons. The legislative effort followed the December 2012 murder of 20 children and six adults by Adam Lanza at Sandy Hook elementary school. The Senate defeated the bill in April 2013.
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