The city of Boulder, Colo., barred assault weapons in 2018 as a way to prevent mass shootings like the one that killed 17 at a high school in Parkland., Fla., earlier that year.

But 10 days after that ban was blocked in court this month, the city was rocked by its own tragedy: Ten people were killed at a supermarket Monday after a gunman opened fire, law enforcement officials said.

In announcing the arrest Tuesday of the suspected 21-year-old gunman – who has been charged with 10 counts of murder in the first degree – investigators determined that the suspect had purchased a Ruger AR-556 pistol March 16, according to the arrest affidavit.

No other details were released as to when or how the suspect obtained the AR-15-style firearm six days before the shooting, or whether the gun was used at the King Soopers grocery store. Police have yet to say whether the ordinance would have prevented him from buying or possessing the weapon within city limits.

Boulder City Attorney Tom Carr declined to comment to The Washington Post, but pointed to language in the city’s code on assault weapons suggesting that the AR-556 pistol linked to the suspected shooter would have been included in the ban that was recently overturned.

Rachel Friend, a city council member, said the events that led to a mass shooting unfolding shortly after a judge blocked the weapons ban left her frustrated and overwhelmed with sadness.

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More about the mass shooting in Boulder, Colorado

“I am still too numb or in shock to say how this happened so quickly on the heels of it being struck down – except to say this is why we wanted to pass the ban in the first place,” Friend told The Post. “It hurts.”

The Colorado State Shooting Association, one of the plaintiffs that sued Boulder over the assault weapons ban, rejected that sentiment, arguing in a statement that “emotional sensationalism” about gun laws would cloud remembrance of the victims.

“There will be a time for the debate on gun laws. There will be a time for the discussion on motives. There will be a time for a conversation on how this could have been prevented,” the group said in a statement. “But today is not the time.”

Though the weapon linked to the suspected gunman is modeled after AR-15-style semiautomatic rifles and includes similar mechanisms, ammunition and magazines, there are some key differences, such as how quickly and easily they can be purchased

AR-15-style pistols have much shorter barrels than their rifle counterparts and don’t use traditional stocks.

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The barrel length for the weapon implicated in Boulder is 10.5 inches, or about half a foot shorter than its rifle cousin. The National Firearms Act regulates short barrel rifles differently than regular rifles because they can be concealed in a bag or under a coat easier than a long rifle, according to a former special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Buying a short barrel rifle requires a lengthy background check that could take months that includes fingerprints, a photo, a purchase from a specialized dealer and $200 tax, according to the former agent, who spoke on the condition of anonymity due to the sensitivity of the issue. But AR-15 pistols aren’t defined as short barrel rifles after gun manufacturers made specific designs to skirt definitions of what makes a rifle a rifle, he said. AR-15 pistols are also deadlier than a typical handgun because they fire more powerful rifle rounds at higher velocity than slower pistol rounds, such as 9mm bullets.

“It’s treated just like a Glock,” he said. “But it’s not just a Glock.”According to the affidavit, witnesses interviewed by authorities said that the suspect had been talking about and playing around with a gun in the days before the shooting.

Weapons like the Ruger AR-556 pistol include a stabilizing brace that helps the operator secure the weapon to their forearm for one-handed firing, like a typical pistol. But the brace can also just be used like a regular stock, as shown in videos, making the pistol functionally identical to the rifle.

The ATF in December sought to examine how a brace affects the classification of a firearm. The agency abandoned its proposal days later after public outcry from gun advocates.

The three-year court fight over Boulder’s ordinance seems likely to preview a similar public debate over whether new gun control measures are warranted after the latest attack in a part of the country that has seen many such incidents. In mourning the 10 victims Tuesday, President Biden called on the Senate to pass two background-check bills already approved by the House and for Congress to reenact an assault-weapons ban.

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The North Central region of Colorado has seen as many as nine school shootings since the Columbine massacre in 1999, which left 12 students and a teacher dead. Four other major shootings have occurred within 20 miles of the high school, including a 2012 shooting at a movie theater in Aurora that left 12 dead.

The earliest of those incidents, as well as the shooting at Marjory Stoneman Douglas High School in Florida in 2018, pushed Boulder officials to take action. Some said they wanted to prevent a similar massacre from occurring again.

“I hope and pray we never have a mass shooting in Boulder,” Carr told the Daily Camera in March 2018. “What this ordinance is about is reducing, on the margins, the ease with which somebody could do that.”

With unanimous support from the council, the law banned the possession, transfer and sale of most shotguns and certain pistols and semiautomatic rifles with pistol grips, a thumbhole stock, or any protruding grip that allows a weapon to be stabilized with the non-trigger hand.

It also established a permit system for people who had previously owned any of those guns and banned large-capacity magazines, which it defined as “any ammunition-feeding device with the capacity to accept more than 10 rounds.”

“If you look at most of the mass shootings, the guns were purchased legally,” Carr said. “I see this as an ordinance that throws in one more barrier to someone who’s contemplating such a horrible act.”

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While city officials had acknowledged the law faced likely legal challenges, they pointed to the city’s home-rule provisions as well as its history of trailblazing on liberal issues, like the issuance of same-sex marriage licenses.

With few steps taken by state or federal government officials, “we had to start somewhere,” Reinfeld said. “When there continues to be mass shootings, when do we take a stand?”

The ordinance generated vigorous opposition from gun rights activists across the state. On the day of the vote, advocates from around Colorado descended on Boulder, many of them carrying concealed rifles with them into city government buildings.

A month after it passed, the law was challenged in state district court by two Boulder residents, a local gun shop and the Colorado State Shooting Association, according to the Denver Post. Richard Westfall, the residents’ attorney, did not immediately respond to a message from The Post early on Tuesday.

On March 12, Boulder County District Judge Andrew Hartman sided with the plaintiffs, saying that, according to a 2003 Colorado state law, cities and counties cannot restrict guns that are otherwise legal under federal and state law.

The “need for statewide uniformity favors the state’s interest in regulating assault weapons,” Hartman wrote. He said Boulder’s ordinance “could create a ripple effect across the state” by encouraging other municipalities to pass their own bans.

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The National Rifle Association cheered the ruling on Twitter last week, noting that its lobbying arm had supported the lawsuit against the ban.

The day after Hartman’s ruling, city officials instructed Boulder police to stop enforcing the ban. Carr, the city attorney, declined to comment on whether he planned to appeal the decision.

But in the wake of the Boulder shooting, gun violence prevention advocates said the importance of preserving such a ban had only become more evident.

Colorado State Rep. Tom Sullivan, a Democrat, who ran for office after his son Alex was killed in the Aurora movie theater shooting, said he helped lobby the statehouse in Denver for background checks and magazine limits. Neither Congress nor the state legislature, he noted, had the political capital to go as far as Boulder City Council.

“The assault weapons put the ‘mass’ in the ‘shootings,’ ” Sullivan, who wore a jacket honoring his late son in the state Capitol on Tuesday, told The Post. “That’s what gets the numbers up. That’s what gets the assault weapons that were able to fire as many rounds as were fired.

“I really want us to not continue to let this happen to communities,” she said to The Post. “We’re reeling and it hurts to be here right now. But I don’t see why we can’t change that for the future.”

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The Washington Post’s Annie Gowen contributed to this report.