In January 2013, police said, a change came over Aguilar. He began researching school shootings, studying anarchy and viewing "violent" images online. In April, Aguilar confided to a general practitioner that he heard voices. McMahon described them as "non-specific, non-violent and really not directing him to do anything."

Police said the doctor recommended the teen see a mental health specialist, and he told investigators he followed up with Aguilar about the issues.

All signs point to Aguilar's failing to heed those instructions. Aguilar kept the issues to himself, writing in his journal that he was reluctant to tell his mother. The general physician told detectives he shared the information with Aguilar's mother, but she disputed that account and police said they have no evidence corroborating the doctor's story. McMahon did not name the physician.

At the College Park house where Aguilar lived with his mother at the time of the incident, no one answered the door Wednesday.

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Johnson's aunt, Sliker, said she believes "the general practitioner did the best job he could given the limitations in place in our society. He did say he talked to the mother. She said she doesn't' remember that conversation. I'm sure in his follow-up he imparted the importance. … Whether he did, we just have to take his word."

As Aguilar's computer research grew to include explosives, the teen also searched for mental illness information, including suicide help.

"It's difficult for us to tell if he took advantage of any of those sites," McMahon said.

In a span of a year before the shooting, Aguilar contemplated killing himself and wrote in his journal that he felt no emotion and was empty and sad. He acknowledged that he needed psychiatric help.

Police found evidence he downloaded a computer game simulating the two Columbine High School shooters, who committed suicide after they killed 12 students and a teacher and wounded 24 others. It was unclear whether he played the game, the police chief said.

All Aguilar's research shifted into "action" Dec. 10 when he paid $430 for a pump-action shotgun that he told a gun shop owner would be used for home protection.

He passed a state-required background check and filled out a state purchase application that asked myriad questions, including whether he had been committed to a mental institution or if he had "ever been adjudicated mentally defective," which would include a court or another legal authority's determining that he was incompetent or a danger to himself or others. The application didn't ask whether he had been observed or treated for a mental illness.

Aguilar checked no, said Dan Millen, a co-owner of United Gun Shop of Rockville.

"If he said anything about seeing anything or about seeing anyone or about a doctor he would have been turned right around," Millen said Wednesday.

The teen came to the gun store alone, but when he returned days later to buy more ammunition, a friend accompanied him, police said. Investigators interviewed the friend and believe he had no knowledge of Aguilar's plan. He thought Aguilar bought the gun for target practice.

Aguilar bought more ammunition Dec. 28 at the Bass Pro outdoor shop in the Arundel Mills store.

On multiple days in January, Aguilar filmed himself taking apart the shotgun, which he outfitted with a pistol grip, a handle similar to at least one of the guns used in Columbine. The video showed his nimbleness taking it apart, which helped investigators understand how he sneaked the 28-inch weapon into the mall in a backpack, McMahon said.

He also videotaped himself making explosives out of fireworks and pellets and was seen on store surveillance Jan. 10 buying a common household cleaner police believe he may have wanted to turn into a bomb.

On Jan. 25, when he was supposed to open a Dunkin Donuts store, he left his College Park home at 5:15 a.m. and was seen on an ATM camera at a PNC Bank in Beltsville at 6:19 a.m.

He boarded a Washington Metropolitan Area Transit Authority bus at 7:31 a.m. and ended up in Burtonsville, where the route ends. At 9:39 a.m., a McDonald's surveillance camera captured him alone at the restaurant. From there he took a cab and was dropped off at the mall alone. He was wearing a light-colored hoodie and gray jacket with a gray backpack, which police say held his disassembled shotgun and explosives. Aguilar walked into an upper-level entrance near the mall's carousel at 10:16 a.m. and moved toward Zumiez.

He didn't go in, but cameras show him riding an escalator down to the food court, where he waited about 43 minutes. At 11:01 a.m., he rode the escalator back upstairs and went into the Zumiez dressing room.