He spent months searching the Internet for mass murder, focusing on the massacre at Columbine, learning how to assemble and fire a shotgun, and how to build a bomb. At the same time, Darion Marcus Aguilar sought out clues to mental illness and joined a chat room filled with people contemplating suicide in their search for a way out.

The 19-year-old from College Park kept all of that private, right up to Saturday, Jan. 25, when at 11:14 a.m. he stood in a dressing room at the Mall in Columbia and snapped a photo of himself holding a Mossberg 12-gauge. He uploaded the image to a blog on Tumblr, adding a note:

“I had to do this. Today is the day. On previous days I tried this I woke up with anxiety, regret and hope for a better future this day I didn’t, I woke up felt no emotions no empathy no sympathy. I will have freedom or maybe not. I could care less.”

A moment later, Aguilar stepped out of the dressing room at Zumiez, a store he had visited before and is popular with the skate and snowboarding sets, and randomly killed two clerks before firing on other mall patrons, wounding one and sending hundreds of panicked shoppers and merchants racing for cover. Then he took his own life.

“He knew he was sick,” Howard County Police Chief William J. McMahon said at a news conference Wednesday, detailing findings of a six-week investigation and revealing a timeline of that fateful day, when Aguilar left home at 5:15 a.m., took a bus to the end of the line in Burtonsville and then used a taxi to get to the mall. He carried a backpack hiding the disassembled shotgun and two makeshift bombs cobbled together with fireworks.

“Nobody saw this coming,” the police chief said.

McMahon said that detectives found no evidence that Aguilar knew the people he killed — Brianna Benlolo, 21, of College Park and Tyler Johnson, 25, of Mount Airy, who worked at the store. Police also said it was pure coincidence that Aguilar and Benlolo lived just blocks apart. McMahon called early false speculation about a romantic connection or that Aguilar was obsessed or angry with one or both of the victims devastating to the mourning families.

But in putting an end to those story lines, police raised a far more frightening one: confirming people’s worst fears that Aguilar intended his attack to be random and far more deadly. The chief said that Aguilar was dressed similar to one of the Columbine gunmen, used a similar weapon and lingered in the mall for 41 minutes to time his attack to the moment when on April 20, 1999, two students walked into Columbine High School, outside of Denver, and began their killing rampage.

McMahon could only speculate that Aguilar stopped shooting because he ran out of targets. Most people in the vast mall had quickly scattered, and Aguilar’s shotgun was not lethal or accurate at long distances. “There could have been a lot more victims,” McMahon said.

Police said that Aguilar fired one shot at Benlolo, who was behind the checkout counter next to the dressing room. Then he shot Johnson several times. Both died immediately, McMahon said.

The gunman then took a shot across the hall, striking one woman in the foot. He aimed another down into the food court, striking the wall next to Great American Cookies and narrowly missing several people. Then, McMahon said, Aguilar turned and shot out the plate glass window of Zumiez, striking a mannequin.

“He then walked back into the store, stuck the shotgun in his mouth and killed himself,” McMahon said. Of the 54 rounds of ammunition Aguilar had stuffed into his backpack, he fired nine. The backpack also held a video camera with images of himself holding the shotgun. McMahon said police would not release Aguilar’s final self-portrait because they believe he intended it as an “effort to gain notoriety.”

Violence erupted at a suburban mall in Columbia, Md. on Saturday morning, after a shooter emerged from among weekend shoppers. Police confirmed that three people were left dead, including one believed to be the shooter.

Police said Aguilar’s family and friends had no idea what he was planning.

Aguilar’s mother could not be reached to comment this week. On Tuesday, at the College Park home where Aguilar lived with his mother, a man walked out carrying two suitcases. “Please just leave my family alone,” he said as he left.

On Wednesday afternoon at the mall, Zumiez remained barricaded by a white wall meant to honor emergency responders and the victims. The memorial was filled with colorful handwriting, ranging from brief notes from sympathetic strangers to sprawling remembrances from close friends.

Visiting the mall for the first time since the shooting, Williams Adams, 60, said the latest information about Aguilar shows that even the most unsuspecting people can harbor dangerous tendencies.

“Anybody could be the perpetrator,” Adams said as he stopped to look at the memorial. “You really don’t know what to look for.”

George Sliker, Johnson’s uncle, said the victims’ families were prepared for what would be released in the report because law enforcement has kept in close contact. Although it has been difficult for the family to relive the memories of the day, he said, the information has helped with the grieving process.

“It’s closing an old wound instead of opening it,” said Sliker, 67. “Every time you talk and open up and grieve a little more, it is a part of the healing process.”

Brian Fischer, a friend of Johnson’s, said the investigation confirmed what he knew all along: that the shooting was random.

“We knew that Tyler could not do anything or cause anything to bring this upon himself,” said Fischer, 34. “Anybody who knew Tyler knew he was an amazing young man. . . . It just saddens me to know that things like this happen to people who absolutely don’t deserve it.”

Aguilar, who graduated in June from James Hubert Blake High School in Silver Spring, had no criminal record. He worked at the Dunkin’ Donuts shop near his house in College Park and was supposed to open the store Saturday morning. Classmates said he did not stand out.

Police said his troubles appeared to surface in January 2013. Over the next year, police said, an examination of his computer showed thousands of Internet queries on school and mall shootings, guns, making bombs and mass murder. McMahon said that the teen downloaded a game in which players can assume the role of a Columbine shooter, although it’s not known whether he played it.

But McMahon said that Aguilar also sought information on suicide and psychiatric issues. He once complained to a doctor of hearing voices but gave no indication that they urged violence, McMahon said.

Police said the doctor recommended that Aguilar see a psychiatrist; there is no evidence that he did so.

The doctor followed up with Aguilar’s mother, McMahon said. The doctor told police that the mother promised to seek help, the chief said, but the mother told police that she didn’t recall talking with the doctor.

McMahon said it was in the few weeks before the shooting that Aguilar acted on his searches, legally buying the pistol-gripped, pump-action shotgun at a store in Rockville and later going back for more ammunition. He was spotted by surveillance cameras in other stores, in one case looking for household cleaner for an explosive, and he bought a strap for his gun online.

Aguilar kept his handwritten journal private as well, and police have previously described the writings as disconnected and violent, including descriptions of him using marijuana and expressing “thoughts of wanting to die.” But police said he never mentioned specific places or people.

On Wednesday, police released a photo of one of the journal entries, in which he scratched: “I going to [expletive] kill you in a couple of hours. I’m anxious, I hate you all so much. You are pathetic pieces of [expletive] who deserve to die.”

Patrick Svitek and Hoai-Tran Bui contributed to this report.

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