Law enforcement sources told the newspaper that weapon was legally purchased on Nov. 30 at Sportman's Warehouse in Tucson.
"This stuff should never happen," store manager Reese Widmier told the Washington Post. "We're cooperating with the ATF and all the federal agencies to help with this tragic incident."
A source was unsure whether Loughner himself purchased the gun, or if someone else did, reports CBS News. A legal sale would involve the purchaser filling out paperwork and a background check to determine if the buyer is allowed to purchase a gun.
A source with knowledge of the gun involved in Saturday's shooting says the weapon used was a Glock 19 9mm semi-automatic pistol.
Importantly, the source said, the gun had a high capacity magazine that can hold up to 30 or more rounds, two to three times a normal magazine capacity; and witnesses said the magazine stuck out about 12 inches. The Glock website confirms that, reading that the standard magazine hold 15 rounds, while the high capacity magazine Loughner had can hold up to 33. One reports says the magazine was probably purchased separately.
The high capacity magazine was banned while the federal assault weapons ban was in place during the Clinton administration; the ban was allowed to expire during the Bush administration.
The gun normally retails for $499, a manager at the Tuscon store told CBS News, while declining to confirm the purchase. Sportman's Warehouse is national chain with stores in 25 states, according to its website.