Darion Aguilar, neatly clad in jeans and a dress shirt, strolled into a Rockville gun store Dec. 10 with a wad of cash and lots of questions. He wanted a weapon for home defense, he told the owners, who remember him as upbeat and courteous. He didn’t know much about firearms and asked for their help in picking one out.

“His whole demeanor was, he smiled, he was polite, he wasn’t aggressive,” said Cory Brown, a proprietor of United Gun Shop. Aguilar, then 18, told Brown and co-owner Dan Millen that he had been researching Mossberg shotguns. Could they show him a Mossberg?

So they got out a basic 500 model — “an entry-level” gun, Brown said — a pump-action 12-gauge that is easy for a novice to fire accurately in close quarters.

Saturday morning, 46 days ­after he left the shop with a $430 Mossberg 500 and two boxes of shells, Aguilar used the weapon at the Mall in Columbia, killing two employees of a clothing store and then himself as hundreds of frightened shoppers ran for cover.

“This guy, to rate him as a customer, he was an ideal customer,” Brown said Monday at his store off Randolph Road. “We get plenty of people that come in here and look shady. We turn them away. We don’t even bother doing the paperwork. But this guy asked a lot of good questions. All ‘please’ and ‘thank you.’ Engaged us great.

Darion Marcus Aguilar (AP)

“Just really good to deal with,” Brown recalled. “Threw up no red flags at all. That’s why I’m so shocked, and I’m waiting to hear what the motive was.

“Because it makes no sense to me.”

As Howard County police continue to investigate the shootings, they said the reason for the attack remains a mystery. They said they have found no connection between Aguilar and his victims, Brianna Benlolo, 21, and Tyler Johnson, 25, who worked in Zumiez, a store for skateboarders, snowboarders and surfers on the Maryland mall’s second level.

Two law enforcement officials, speaking on the condition of ­anonymity because the investigation is ongoing, said Aguilar kept a journal in which he described suicidal thoughts. When the young man’s mother reported him missing Saturday, they said, a police detective was sent to the home. He began reading the journal, but Aguilar’s mother demanded he stop.

Later, after authorities identified Aguilar as the shooter, police seized the journal. In addition to the references to suicide, it contains notes expressing hatred of certain groups, according to the officials, who did not elaborate in detail.

Aguilar, a 2013 graduate of James Hubert Blake High School in Silver Spring, shared a home with his mother in College Park, not far from Benlolo’s residence. But there’s no evidence the two were acquainted, police said.

Johnson, the other victim, lived in Mount Airy.

Police said mall surveillance video shows Aguilar lingering in the first-level food court for about an hour after he arrived at the shopping complex by taxi about 10:15 a.m. Then, toting a backpack, he rode up an escalator and apparently walked into a dressing room in Zumiez. Moments later, he emerged with the shotgun and opened fire.

He easily could have disassembled the 35-inch-long Mossberg beforehand, stashed the parts in his backpack and reassembled them in the dressing room, Brown said.

“Very quickly,” he said “All it takes is a simple screwdriver.”

With help from Zumiez’s management, police are reviewing receipts and surveillance video to determine whether Aguilar had visited the store before the shootings, they said.

At United Gun Shop that Tuesday last month, Brown noticed not only Aguilar’s crisp dress shirt and jeans but also his shoes — a pair of black, DC-brand skateboarder’s sneakers. Brown said his 7-year-old son has asked for just such footwear because many of the boy’s skateboarding friends wear them at school.

Aguilar “said they were really comfortable,” Brown recalled. “He said you can wear them with jeans, wear them with khakis.” But he didn’t say where he got them.

Ellis Cropper, a spokesman for the Aguilar family, said that the dead gunman’s mother, Jordan Aguilar, did not know the two victims and hadn’t heard their names before news of the shootings broke Saturday.

Cropper, a friend of the Aguilars, said the killings stunned him. His children had gone bowling with Darion Aguilar this month, he said. When he learned what Aguilar had done at the mall, Cropper said, he thought: “No way. It cannot be.”

At 5-foot-10, Aguilar weighed just 130 pounds, according to his driver’s license. He worked at a Dunkin’ Donuts in Prince George’s County. Cropper said Aguilar was health-conscious and wanted to study environmental engineering. After watching a documentary about the slaughter of animals, he swore off eating meat, Cropper said.

One of his high school friends, Francisco Xavier Lendor, said Aguilar grew distant after their graduation last spring. “I tried messaging him on Facebook . . . and joked with him on Instagram about him giving me free doughnuts,” Lendor said.

“But after that, we never kept in touch,” he said. “From what I hear from our shared friends, he was fine. He was still the same Darion we all knew and loved.”

In Rockville, Brown, 39, also had difficulty reconciling the mayhem in the mall with the chipper, inquisitive young “preppy” he met in his gun store seven weeks ago.

Before leaving with his shotgun, Aguilar presented his driver’s license, bearing his old Silver Spring address, and a state-issued change-of-address card listing his College Park residence. He gave Brown his Social Security number and filled out a federal firearm purchase form. And he passed an instant background check.

Brown said Aguilar told him that he’d seen the sign for United Gun Shop while driving to a friend’s home nearby and decided to stop in.

“His whole context was home defense,” Brown said. “He was new to this. He wanted something not too crazy — he didn’t ask about [assault] rifles; he didn’t ask about handguns. He said he had a limited budget and didn’t need any of that tactical stuff.”

The Mossberg 500 was perfect for Aguilar’s stated needs. “He said, ‘I want something to learn with,’ ” Brown recalled.

He said the model that Aguilar bought, paying cash, has an 181 / 2-inch “home defense barrel,” designed for easy maneuverability and shooting in tight spaces. The gun is made of blue steel and hard black plastic. It holds six rounds, five in the ammunition tube and one in the firing chamber.

When investigators found the weapon near Aguilar’s body, they said, it had a pistol grip, though it didn’t have one when he purchased it.

“We don’t sell them,” Brown said. The gun had a conventional stock. “One screw, though, the stock comes off, and it’s a $10 grip you just stick on there.”

As for ammunition, Aguilar bought a box of 10 Winchester birdshot shells for $9 and paid $16 for a box of 10 Hornady “Critical Defense” buckshot shells, Brown said.

Birdshot is for killing small animals. Buckshot is for bigger game. And the buckshot that ­Aguilar bought is meant to be used for self-protection.

“It’s designed for close-quarters home defense,” Brown said. “The whole idea is, you’re going to hit your target, but there’s not enough energy in the shot to go through walls and hurt other people. If you have kids or other people in the house, and you miss the intruder, the chances of it going through the Sheetrock and killing someone else is minimal.”

Aguilar returned about 10 days after buying the weapon and bought another box of birdshot, saying he’d used up the first box in target practice, Brown said. Police said they found a large quantity of shotgun ammo with Aguilar’s body. But they did not specify what type of shells he used at the mall.

Brown said Aguilar “was really seeking to understand what he was buying. He even asked about safety. He was like, ‘How do I store this?’ And we showed him about gun locks, proper storage of ammo, keep it apart from the gun. If you travel with it, here’s the proper way.

“He asked lots and lots of questions, all the right stuff.”

Ian Shapira and Lynh Bui contributed to this report.

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