Police say the Boulder shooting suspect bought a gun 6 days before the attack. There's no waiting period before buying a gun in Colorado.

• The Colorado shooting suspect is said to have bought an AR-556 pistol days before the attack.
• The 21-year-old man is accused of fatally shooting 10 people at a grocery store in Boulder.
• Just 10 states in the US have laws requiring a waiting period before a person can buy a gun.

The police said Tuesday that the man charged with killing 10 people at a grocery store in Colorado bought a semiautomatic weapon less than a week before the attack.

Ahmad Al Aliwi Alissa, 21, is accused of opening fire at a King Soopers grocery store in Boulder on Monday afternoon, killing 10 people, including a police officer.

Alissa purchased a Ruger AR-556 semiautomatic pistol on March 16, according to an arrest warrant for Alissa released by the Boulder Police Department on Tuesday. It wasn't immediately clear whether that gun was used in the shooting; a police affidavit included in the warrant said an AR-15-style rifle as well as a semiautomatic handgun were recovered at the scene.

According to the arrest warrant, a woman, whose identity is redacted in the documents, who is married to Alissa's older brother told the police that Alissa was "seen playing with a gun she thought looked like a 'machine gun'" about two days before the shooting.

At the time, Alissa told the woman there was a bullet stuck inside it, she said, according to the affidavit. Two people present, whose identities were likewise redacted from the arrest warrant, were "upset" with the way Alissa was "playing with the gun" and took it from him, the woman told the police.

There is no waiting period between when people in Colorado first express interest in acquiring a gun and when they're able to buy one

Just 10 US states and Washington, DC, have some sort of law requiring a waiting period between when a person attempts to purchase a gun and when the person is actually allowed to take possession of that weapon, according to the Giffords Law Center. The longest waiting period - 10 days - is required by California and in the nation's capital.

There is also no waiting period in Georgia, where last week a man opened fire on three spas in and near Atlanta, killing eight people. The suspect in that case bought the gun the same day as the shootings.

"Obviously that cannot prevent all acts of violence or self-harm, but there is good research that shows that it does lead to reductions in both suicide and homicide by preventing some impulsive, heat-of-the-moment acts of violence," said Ari Freilich, the state policy director at the Giffords Law Center.

Freilich told Insider these laws were meant to help thwart sudden impulses to commit violence and provide more time for law enforcement to conduct background checks. They also give families and friends more time to recognize a person is in a "crisis period" and intends to cause harm, allowing them to preemptively call law enforcement or mental-health professionals, he said.

According to the arrest warrant, Alissa had a prior charge of third-degree assault in the nearby city of Arvada, Colorado, though it's not clear whether he was ever convicted of the charge.

If Alissa legally purchased the semiautomatic pistol from a firearms dealer, a background check would have been required under federal law, according to the Giffords Law Center.

Under Colorado law, a private seller is also legally required to use a Federal Firearm Licensee to check the background of anyone purchasing a firearm, unless the transaction is between family members.

Freilich said Colorado had some of the stronger background-check laws in the US, as they apply to most sales, including from private sellers, with "narrow exceptions" that apply to family members or a "temporary loan" of a weapon.

Haven Orecchio-Egresitz contributed reporting.