Darion Aguilar

A senior yearbook photo of Darion Aguilar from the Blake High School, Silver Spring, MD yearbook. Aguilar has been identified as the shooter at Columbia Mall who killed two store employees on Saturday. (Handout photo / January 26, 2014)

Dan Millen remembers Darion Marcus Aguilar walking into his Rockville gun shop with an air of confidence and a wad of cash. Aguilar wanted a shotgun, which he said he planned to keep in his home for protection.

Aguilar's dress was nondescript, Millen said, except for a pair of high-top skateboarding shoes. The 19-year-old asked to see a Mossberg 500 series 12-gauge, a basic pump-action model used for sport, bird hunting or self-defense.

He passed a state-mandated FBI criminal background check within minutes, paid $430 and walked out of the store Dec. 10 with the shotgun, which authorities say he used Saturday to kill two employees at a skate shop in The Mall in Columbia before taking his own life.

"It never settled in to me that I would be selling the gun to someone who would commit a crime," said Millen, noting that the transaction was legal. "I feel horrible for the two people. I wish I could have gone back that day and really dug in."

Millen gave the account as a fuller picture emerged Monday of Aguilar, who police say killed Brianna Benlolo, 21, and Tyler Johnson, 25 — both workers at Zumiez. Howard County investigators say they still don't know enough about Aguilar to determine what motivated the attack.

Aguilar's parents married in Denver in 1992, and his mother appears to have lived there for a few years before moving to Maryland when Aguilar was young, according to divorce records. He would later move to Maryland, and at the time of the shooting was living in College Park with his mother.

Aguilar's parents were separated for 14 years before they divorced last October, according to papers filed in the Circuit Court of Prince George's County. He was one of two children, and a family friend said his mother, Jordan C. Bullard Aguilar, works in medical management. Aguilar has a sister nearly two years older.

His father, Ronald Matthew Aguilar, lists a St. Louis address in divorce records. A woman who said that she was one of his family members declined to comment Monday.

Public records show that Aguilar lived for five years at an address in Silver Spring owned by a surgeon, Elwood McGee. Reached by phone, McGee confirmed that Aguilar and other relatives lived at the home.

"They lived here before. They moved out," McGee said. "I don't have any insight, and I don't have anything new to add. They say he was a nice kid — I agree, he was a nice kid."

He declined to elaborate.

Like many others who knew or interacted with Aguilar, Millen agreed that the young man presented himself well. Nothing set off any "red flags," said Millen, who opened the United Gun Shop with a business partner about 10 months ago.

Millen said he enjoys shooting clay pigeons during the weekends and is enthusiastic about the benefits of responsible gun ownership.

Aguilar said he wanted the gun to protect his home after moving from Silver Spring to College Park. The teen didn't know anything about guns, Millen said, but he knew he wanted a Mossberg 500.

The gun, Millen said, is known as an "inexpensive tried-and-true gun."

First made in 1961, more than 10 million of the pump-action shotguns have been manufactured by Connecticut-based Mossberg. One of the series' models has been the "shotgun of choice for the U.S. military" over the last 30 years, outdoor writer Brad Fitzpatrick wrote in the October edition of Guns & Ammo magazine.

The guns are fast and reliable with a "reputation for dependability," Fitzpatrick wrote. It is not known to malfunction often and can almost be as fast as a semiautomatic in experienced hands.

Maryland law does not place the same restrictions on hunting weapons such as shotguns as those that govern other weapons. A law passed last year banned the sale of assault rifles and high-capacity magazines, and some said that change may have prevented Saturday's shooting from being even more deadly.

But gun-control advocates said Saturday's shooting was unlikely to reignite the debate after last year's changes.

"It's very unlikely that anything will pass that will add on to all of the protections from last year," said Sen. Brian Frosh, a Montgomery County Democrat who shepherded the gun-control legislation through his chamber. "People have lost their energy for it."