SANTEE, California (CNN) -- Two teen-agers are dead, 13 people are wounded and a 15-year-old "who was picked on all the time" faces murder charges as residents of this San Diego suburb struggle to understand why another American high school was transformed into a killing zone.

"I know in your minds is the overriding question: 'Why?'" said San Diego County District Attorney Paul Pfingst. "The suspect has made statements. I will not share the contents of the statements with you at this time, but there is no real answer. I am not sure ... we will ever know why."

The gunfire erupted about 9:20 a.m. Monday (12:20 p.m. EST) at Santana High School, about 10 miles northeast of San Diego. Law enforcement authorities suggest Charles Andrew Williams, known to other students as Andy, fired the .22-caliber handgun randomly "30 or more" times and reloaded "at least four times."

Lt. Jerry Lewis of the San Diego County Sheriff's Department said officers believe the incident began when Williams entered the boy's bathroom in the 200 wing of the high school and shot two people.

The boy then opened the bathroom door "and started shooting, randomly, anybody who was passing by. He did not have any particular targets in mind, anybody who passed by, from the information we received, he was shooting at," Lewis said.

Suspect under observation

Authorities and witnesses said Williams had talked over the weekend about taking a weapon to school, but that he later said he was joking.

"He was telling us how he was going to bring a gun to school ... but we thought he was joking," 15-year-old Neil O'Grady said. "We were like, 'Yeah, right.' "

"He always gets picked on. He's scrawny, he's little," O'Grady said. "People think he's dumb."

"He was picked on because he was one of the scrawniest guys," said student Jessica Moore. "People called him 'freak,' 'dork,' 'nerd,' stuff like that."

Authorities said one student died at the scene and another died at an area hospital. They were identified as Brian Zuckor, 14, and Randy Gordon, 17. The wounded were 11 students and two adults.

Williams was isolated and under observation at San Diego County Juvenile Hall, an official said.

The boy underwent a preliminary medical and mental health evaluation, standard for all juvenile detainees in the state, the official said. He would not discuss the results of those evaluations.

'Evil kind of sadistic demeanor'

When the shooting began, the school -- which employs security guards but does not have metal detectors -- was evacuated and a police SWAT team swept the building.

Student Alicia Zimmer said, "I was probably about 10 feet away from some of the victims ... I saw a boy laying on the floor with his face downward" and a girl with "blood all over her arm."

"Then all of a sudden, we heard more shots going off," she said. "It sounded more like a cap gun than anything. It was really scary. Everybody was running."

Student John Schardt said Williams, with the revolver in his hands, smiled as he emerged from a restroom in the school's social sciences building. (see video)

"He was looking around, smiling, with his weapon. He fired two more shots and went back in," Schardt said. He said authorities -- with guns drawn -- arrested the boy in the bathroom.

"He had an evil kind of sadistic demeanor to him," Schardt said.

Andrew Kaforey, a 17-year-old senior, said he ran into the bathroom with a security guard after hearing what sounded like a firecracker or a gunshot. "He pointed the gun right at me but he didn't shoot," Kaforey said.

As he and the guard ran out, Kaforey said the guard was shot in the back.

San Diego Sheriff's Department Deputy Ali Perez said Williams offered no resistance when he and other officers closed in. As officers asked whether there was another shooter, Williams looked up, gestured to surrender the pistol, and said, "It's only me," Perez said.

Suspect 'joked' about plans

Monday's attack was the nation's deadliest school shooting since the April 1999 bloodbath at Columbine High in Littleton, Colorado, where two teen-agers killed 12 fellow students and a teacher before committing suicide.

"The whole weekend he was talking about it," Josh Stevens said, "and me and my friends were like, 'You're not serious, are you?' And he says, 'I'm just joking.' And then he asked us if we want to do it with him.

"And we were all, 'You're joking.' He was like, 'I am, I'm just messing around,' " said Stevens, who was questioned by authorities after giving that account to KGTV television.

Chris Reynolds, who identified himself as the boyfriend of Josh's mother, said he had heard Williams and other friends "talking about ... and kind of like joking about ..." the possibility of a shooting at the school.

Reynolds said that Williams' mother lives out of state. He said the boy stayed at Reynolds' house Saturday night and talked about starting a shooting spree.

"Sometimes I kind of took them seriously, the way they were talking, and then other times it seemed like they were just joking because these kids like to joke a lot," Reynolds said.

"I even mentioned Columbine to him. I said I don't want a Columbine here at Santana. But he said, 'No, nothing will happen, I'm just joking,"' Reynolds said.

'I kinda feel like I'm to blame'

"I kind of feel like I'm to blame for some of this because I could have done something," Reynolds added. "And I've had kids before I've known get hit by drunk drivers and things like that, and here's where I could have done something to prevent it."

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Sheriff's and FBI officials descended on the apartment where Williams lived with his father. Under authority of a warrant they entered the home and later removed a computer hard drive.

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The federal Bureau of Alcohol, Tobacco and Firearms said local law enforcement officials have asked them to assist with the investigation.

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Federal authorities were trying to trace the revolver that the suspect dropped as he was taken into custody, said U.S. Deputy Attorney Sandy Jones.

No family members had visited Williams in custody, an official said Monday night. The visitors' log showed that two people who identified themselves as public defenders -- Henry Coker and Randy Mize -- signed in to see Williams, though there was no indication that they met the boy.

The Associated Press contributed to this report.

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