By Amir Vera, CNN

(CNN) -- We still don't know why a nearly-naked gunman unloaded his assault-style rifle at a Waffle House, killing four people Sunday morning.

But more details are emerging as the suspect is now in custody. Here's what we know, and what remains a mystery:

What happened?

Police say Travis Reinking, 29, approached a Waffle House in Antioch around 3:23 a.m. (4:23 a.m. ET) and used an "assault-type rifle" to fatally shoot two people standing outside, Metro Nashville police said.

Reinking, who was barely clothed at the time, then went inside the restaurant and continued firing, police said. Two victims inside were also fatally shot.

The shooting ended when a customer, James Shaw Jr., wrestled the weapon away from the gunman, who then fled on foot.

After a frantic manhunt, police arrested Reinking on Monday.

Who is the suspected gunman?

Reinking is from Morton, Illinois. He moved to Tennessee in 2017.

According to police, Reinking was once arrested by the Secret Service for trespassing near the White House. He was charged with unlawful entry, an arrest report states, but had his charges dismissed after completing community service.

The FBI interviewed Reinking shortly after he completed the program and seized his guns.

Authorities in Tazewell County, Illinois, later returned the seized weapons to Reinking's father, who gave them back to his son, police said. One of those guns was the same AR-15-style rifle used in Sunday's attack.

In May 2016, Reinking experienced a delusional episode in his hometown of Morton, according to a police report. He told first responders that he believed pop star Taylor Swift was stalking him. Reinking's family also told police he had made comments about killing himself and owned several guns.

Who are the victims?

The four people killed were all under 30 years old.

Two of them were fatally shot outside the restaurant: Waffle House employee Taurean C. Sanderlin, 29, and customer Joe R. Perez, 20.

Inside, the gunman killed two more people: 23-year-old Akilah DaSilva and 21-year-old DeEbony Groves.

DaSilva's cousin said the world lost a talented young man who excelled in computer science and music.

"He was a genius. He was super smart," Kareem DaSilva said. "One of the things I liked about him is that he was a hard worker. He never gave up, and he was quick to learn. He was very impressive."

What happened to the man who stopped the shooting?

The man who wrestled the gun away from Reinking and prevented him from killing more people doesn't want to be called a hero.

James Shaw Jr. said he ambushed and wrestled Reinking to save himself.

"I'm a believer that everybody could do what I did," Shaw told CNN affiliate WSMV-TV.

Shaw tackled Reinking as he was looking at his rifle and had stopped shooting.

Shaw was grazed by a bullet on his elbow while grappling with the gunman. He also burned his right hand grabbing the barrel of the weapon.

The shooting didn't stop Shaw from going to church with his father Sunday morning, hours after confronting the gunman.

Shaw also created a GoFundMe page Sunday to assist the victims of the shooting, a GoFundMe spokeswoman told CNN. Within hours, the $15,000 goal had almost been met.

Correction: A previous version of this story incorrectly identified Taurean C. Sanderlin as a woman.