HIGHLANDS RANCH, Colo. (AP) — Two Colorado high school students used at least two handguns to attack the charter school they attended, killing one of their classmates and wounding eight others in an assault near Columbine High School, authorities said Wednesday.

Because the attack at STEM School Highlands Ranch happened only miles from Columbine and just weeks after the shooting's 20th anniversary, questions quickly arose about whether it was inspired by the 1999 massacre. But investigators offered no immediate motive.

Douglas County sheriff's officials said 18-year-old Devon Erickson and a younger student who is a juvenile and was not identified walked into the STEM School Highlands Ranch through an entrance without metal detectors and opened fire in two classrooms.

Sheriff Tony Spurlock told reporters that the suspects had a "number of weapons," including two handguns they were not old enough to buy or own.

Authorities on Tuesday identified the juvenile suspect as a boy, but Spurlock on Wednesday told reporters that the suspect is a girl.

"We originally thought the juvenile was a male by appearance," Spurlock said.

He declined to comment when asked by reporters if the person is transgender.

Television station KMGH reported late Tuesday that the juvenile suspect is a transgender male in the process of transitioning from female to male. The station cited anonymous sources close to the investigation.

The student who was killed was 18-year-old Kendrick Castillo, authorities said.

Student Nui Giasolli told NBC's "Today" show that she was in her British literature class when Erickson came in late and pulled out a gun.

Castillo lunged at the gunman, who shot the teen. Castillo's actions gave the rest of the class time to get underneath their desks and then run across the room to escape to safety, Giasolli said.

Rachel Short said Castillo was a funny and empathetic person who loved others and was a part-time employee at her manufacturing company, Bacara USA.

"To find he went down as a hero, I'm not surprised. That's exactly who Kendrick was," Short said.

Cecilia Bedard, 19, knew Castillo since elementary school and said he was always friendly, modest and excited to help people. He made a point of always joining his father at Knights of Columbus fundraisers and bingo nights.

"He was amazing," Bedard said. "He was honestly the sweetest kid I ever met. Never said a mean joke."

Both suspects were students at the school, and they were not previously known to authorities, Spurlock said.

Josh Dutton, 18, said he was close friends with Erickson in middle school but had not seen him for four years as he attended a different high school. On Sunday, he spotted Erickson at a local light rail station and said he was shocked at how much his friend had changed.

Erickson wore all black, a hat and sunglasses, was significantly thinner and did not seem interested in talking.

"He said he'd just turned 18 and he owned rifles," Dutton said.

A message left at a phone number listed for Erickson's home was not immediately returned. He was scheduled to make an initial court appearance later Wednesday.

It was not immediately clear when the younger suspect, who has not been identified, would appear in court.

The shooting took place exactly a week after a gunman killed two students and wounded four at the University of North Carolina-Charlotte.

It also came nearly three weeks after neighboring Littleton marked the anniversary of the Columbine attack that killed 13 people. The two schools are separated by about 7 miles (11 kilometers) in adjacent communities south of Denver.

Douglas County District Attorney George Brauchler said the community remains resilient in the face of multiple shootings, including Columbine, the 2012 theater shooting in the Denver suburb of Aurora and the 2013 shooting at Arapahoe High School.

The attacks are "aberrant acts" although they might seem otherwise to the rest of the world, he said.

"Who we are is a kind, compassionate, caring people, and this does not define us. It won't today and it won't tomorrow," he said.

Three hospitals reported treating eight people in connection with the attack. Five had been released by Wednesday morning but three remained hospitalized in intensive care, Spurlock said.

STEM is a public charter school with a focus on science, technology, engineering and math. It has more than 1,850 students in kindergarten through 12th grade.

As gunfire echoed through the school, students hid or ran through the halls, some shouting. Frantic parents used their cellphones to find their children as news of the shooting spread.

"No one really knew what was going on so I didn't know they were bullets," said seventh-grader Sophia Marks. "I just kind of saw like flashes and we heard bangs."

Within minutes, deputies at a nearby sheriff's department substation entered the school and arrested the two suspects without exchanging gunfire. One of the suspects was detained by the school's security guard, Spurlock said.

The security guard was employed by Boss High Level Protection, a company started by a former SWAT team leader who responded to the Columbine shooting. The owner, Grant Whitus, told The Associated Press the security guard is a former Marine who ran to the area of the shootings and confronted one of the armed students in a hallway.

The guard drew his weapon and apprehended the person, Whitus said.

"He doesn't even realize how many lives he saved by stopping a school shooting," Whitus said.

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Riccardi reported from Denver. Associated Press writers Dan Elliott and Colleen Slevin in Denver and AP researchers Monika Mathur in Washington and Rhonda Shafner in New York contributed to this report.