A gold pistol sits at the center of a display of rare and collectible handguns Tuesday at Northwest Armory in Portland, Ore. The high-end gun store sells handguns, hunting rifles and more to customers willing to go through a background check. David Gilkey / NPR

Law enforcement agencies in Multnomah County on Friday began investigating firearms sales that were denied after a background check.

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While the numbers are small, police say it's critical to follow up with people who fail background checks.

"So, you've got a felon, who knows they are a felon walking into a store trying to buy a firearm," said Multnomah County Sheriff Mike Reese. "There's not the accountability for the person that's trying to purchase it."

The Portland Police Bureau, Multnomah County Sheriff's Office, Gresham Police Department, Oregon State Police and the District Attorney's Office announced Friday they would work together to prevent illegal sales of firearms in the county.

Twelve law enforcement agencies in Oregon already partner with OSP to investigate attempted purchases by felons.

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"This partnership will allow us to have a district patrol officer immediately dispatched to that transaction location and to conduct an on-scene investigation that they'll forward to the District Attorney's Office," Reese said.

Roughly 20,000 guns are sold in the county every year.

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Most purchases are subject of a background check by Oregon State Police. About 200 attempted purchases are denied each year because the buyer is considered a prohibited person. That group includes felons, domestic abusers and some people suffering from mental illnesses.

Until now, OSP has been responsible for investigating denied sales in Multnomah County.

"In this new endeavor, we're going to pick that work up for them," Reese said.

Last year, Oregon Gov. Kate Brown ordered OSP to notify law enforcement agencies if a person who is prohibited from buying a gun tries to do so.

"While the percentage of purchase denials are relatively small, the public has an expectation law enforcement will supply quick and efficient investigations of prohibited persons attempting to obtain firearms," Oregon State Police Superintendent Travis Hampton said in a statement Friday.

Multnomah County District Attorney Rod Underhill could charge people who try and buy guns illegally with various misdemeanors, including unlawful possession of a firearm and providing false info in connection with a firearm transfer.