Many now know Scott Ostrem as the man who, police said, walked into a Walmart store in Colorado, calmly pointed a firearm, shot random customers, turned around and left.

The 47-year-old Denver man was called names on social media. "Walmart terrorist." "Another angry white male shooter." "Violent leftist."

But to at least one person, his stepsister, Ostrem had a deeper, darker problem that drove him insane for three decades and likely manifested the evening of Nov. 1 when, police said, Ostrem shot and killed two men and a woman inside a Walmart store in the Denver suburb of Thornton.

That problem began at a drug party in 1988 when Ostrem, then in his late teens, ingested an excessive amount of LSD, a powerful psychedelic more commonly known as acid, according to Michelle Willoughby. Ostrem had been hearing voices telling him that the devil was coming for him since that bad LSD trip, Willoughby said.

Willoughby had not responded to a Facebook message from The Washington Post on Tuesday, Nov. 7. Efforts to reach her by phone also were unsuccessful. But in an interview with the Denver Post, she said the 16 doses of LSD that her stepbrother ingested that night changed him from an outgoing person who played sports and loved the outdoors to a recluse constantly haunted by demonic voices.

"My brother is not this monster. He is not cold blooded. He hears voices. Honestly, in my heart, I believe there is only so much a person can take," Willoughby told the Denver Post.

She said Ostrem came home after the party saying that he was hearing voices.

"My brother was freaking out," she said. "He was screaming that the devil was after him."

For the next several years, Ostrem was regularly visited by a priest who read Bible verses to him. Willoughby said she had seen the priest place a crucifix on Ostrem's forehead as he commanded demons to leave. At one point, Ostrem told Willoughby that she was 150 years old, and she went along with it, she told the paper. Later, Ostrem quit his job and moved to the mountains. Willoughby didn't see her brother for years, until she saw pictures of him after he was arrested this week.

She said her brother had never had a mental evaluation.

Willoughby, who's from Colorado but now lives in Florida, said she has since retreated to a boat "in the middle of the [Atlantic] Ocean" after she received death threats for defending Ostrem on social media.

A spokeswoman for the 17th Judicial District Attorney's Office in Colorado has not responded to a call seeking comment about Willoughby's comments to the Denver Post.

Ostrem was arrested Thursday morning after an overnight manhunt. Police picked him up from his Denver apartment after receiving a tip from a citizen.

An image from a surveillance video taken from the Walmart store's entrance shows a slim man with thinning hair and a receding hairline, both of his hands hidden in the pockets of his black jacket.

"From the witness accounts, he nonchalantly walked into the front of the business through the south door and just raised the firearm and began firing," Thornton police spokesman Victor Avila told reporters at a news conference Wednesday night.

Ostrem then drove away in a red, four-door hatchback, police said.

Two of the victims, Carlos Moreno, 66, of Thornton and Victor Vasquez, 26, of Denver, died immediately. Pamela Marques, 52, of Denver died later at a hospital, authorities said.

Investigators have not released any information about a motive for the shooting.

Some of Ostrem's Hispanic neighbors told local media that he was hostile toward them.

"Sometimes I would tell him good morning, and he would look at me and tell me to F- off," Dennis Valenzuela, 49, told NBC affiliate KUSA.

Ostrem is facing 36 counts, authorities said, including murder and attempted murder.

The Walmart shooting happened just three days before a lone gunman stormed into a small community church in rural Texas and fatally shot more than two dozen parishioners, many of whom were children.

Authorities said Devin Kelley, 26, was dressed all in black, wearing a tactical vest and armed with a rifle as he approached First Baptist Church in Sutherland Springs, a country town about 30 miles southeast of San Antonio.

A man who lives near the church and heard what was happening fired his own rifle, striking Kelley at least once, authorities said. Kelley then fled and was later found in his Ford Expedition SUV. Authorities said he had shot himself in the head.