The new system, which supersedes the FBI program, will only affect people who purchase firearms from licensed gun dealers.
Inspector Pete Mang with the CBI said the reinstatement of state checks will close some, but not all, loopholes in the federal National Instant Criminal Background Check system.
"The state has more complete and detailed information on restrictions including restraining orders which will help prevent those people who have criminal histories from legally purchasing firearms," he said.
Spurred by the fatal shootings of three girls by their father in June, Gov. Bill Owens signed an executive order mandating that the state take over the instant background-check program that had been operated by the FBI since April.
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Simon Gonzales purchased a handgun from a federally licensed gun dealer only hours before he killed his three daughters while they were sleeping. He then died in a shootout with Castle Rock police.
It was only after the shootings that it was discovered his estranged wife had filed a restraining order against him, something the federal checks did not uncover and that could have prevented the purchase.
By 5 p.m. on its first day of background checks since April, the CBI had reviewed 68 requests and rejected eight bids to buy guns, two of which it said would not have been denied by the FBI.
One request was denied because of a restraining order, another because of a felony charge, which the FBI check would not have caught because the potential buyer was not fingerprinted at the time of arrest.
The new Colorado system still has loopoles, Mang said.
Gun sellers who are not federally licensed, including private sellers who frequent gun shows, are not required to do background checks. Neither are individuals who sell guns privately, either through newspaper ads or over the Internet.
"Of course it's not a panacea," Mang said. "If someone wants to buy a gun illegally, it is easy to do so. But this provides another level of protection, and that might prevent someone else from slipping through the cracks."
Another major loophole the state system does not address is the ability of people with mental illnesses to purchase guns.
Federal law prohibits people with serious mental problems from buying guns but there is no reliable list of those people in Colorado. Colorado law prohibits even asking for someone's mental health records.
For licensed gun dealers, the return to a state-controlled system is not especially arduous.
"It's basically just a different phone call and just one more piece of paper to fill out," said Kevin Schwartzberg, the manager of Firing Line, an Aurora gun shop and shooting range. "It's no more or less of an inconvenience."
The next logical step, according to student activist David Winkler, would be to extend the regulation to gun shows.
The 17-year-old and a friend walked into the Tanner Gun Show, an exhibition of firearms that stops in Colorado 10 times a year, and purchased a TEC-DC9, the same semiautomatic weapon used by the teen-age gunmen in the April 20 massacre at Littleton's Columbine High School.
"It just proves how important it is to end unrestricted anonymous purchases, to have any gun purchase cleared with law enforcement," said Winkler, who is a member of Safe Alternatives to the Firearms Epidemic.
Under the current bill, the state will absorb the costs of the firearms checks.