UPDATE: (CNN) -- When the shooting started at her high school near Portland, Oregon, early Tuesday, student Jaimie Infante didn't recognize the sound of the gunshot.
She thought maybe somebody had dropped a book.
In reality, a lone gunman had opened fire at the school, killing one student and forcing others to flee.
"At the end he said: 'This is not a drill,'" Infante told CNN.
A teacher suffered non-life-threatening injuries. He was treated at the scene.
The suspected gunman is also dead, the Multnomah County Sheriff's Office said.
According to multiple law enforcement officials, the shooter was a student at the school. The gunman appears to have died from a self-inflicted wound, the sources told CNN.
Authorities are working to notify the relatives of both the shooter and the student, neither of whom has been identified.
Officials believe they know who the gunman was, but they are "not confident enough" to reveal details yet, Troutdale Police Chief Scott Anderson told reporters midday Tuesday.
"Today is a very tragic day for the city of Troutdale and Reynolds School District," he said. "I'm very, very sorry for the family, and for all the students, and everybody who will be impacted by this tragic incident."
About an hour after the shooting, Oregon State Police said the area was secure and the situation was contained.
The shooting happened at about 8 a.m. (11 a.m. ET) at Reynolds High School in Troutdale, about 12 miles east of Portland. The city has a population of about 16,400 people.
Classes were in session at the time. Details about what led to the shooting weren't immediately available. The gunman is believed to have used a rifle.
A couple of SWAT teams responded, said Sue Strickland of the Troutdale Police Department.
"Without the quick and well-executed response, this tragedy could have been much worse," the Reynolds School District said in a statement.
Anxious parents waited for news that their children were safe. One couple, Craig Tuholski and Tawnjia Reimer, were talking with CNNaffiliate KGW, about how agonizing the wait for news was, when Tuholski's cell phone rang.
"Is that Chris?" Reimer asked before letting out a sigh of relief. "Oh, thank God."
"That's what we were waiting for," Tuholski said after hanging up.
A prayer vigil is being planned Tuesday night at Greater Portland Baptist Church.
CNN first learned of the shooting through reports on Twitter.
"My heart is heavy after learning of this morning's tragic events at Reynolds High School," said Oregon Gov. John Kitzhaber. "My thoughts and prayers are with students, staff and the Reynolds High School community."
TROUTDALE, Ore. (KOIN 6) - A gunman and one Reynolds High School student are confirmed dead following a shooting inside the Troutdale school. Law enforcement officials offered few other details concerning the shooting during a 10 a.m. media update.
"This is a very tragic day, one that I hoped would never ever be part of my experience," Reynolds School District Superintendent Linda Florence said to parents, students and media.
Around 100 law enforcement officers, tactical teams and medical personnel are in the process of evacuating students and staff.
No one else is known to have been injured in the shooting.
One student told a parent the shooting began in the school's gymnasium, though police have not been able to confirm specific details.
Another student interviewed at 10:30 a.m. suggested there was more than one shooter inside the school, though police have not confirmed any additional gunman.
The student did say he heard two shots and witnessed a shooter, who he said was male and was not covering his face. The student could not say who the shooter was.
Law enforcement officials confirmed the known shooter was likely armed with a rifle.
Police were first alerted to the shooting just after 8 a.m.
Parents are being asked to reunite with their children at the Wood Village Fred Meyer at NE 223rd and NE Glisan, not the Safeway location initially given.
Parents of Reynolds High School students anxiously await information. (KOIN 6) A student still within the school tweeted just after 9 a.m. that while some students had been evacuated, "most" were still on lockdown.
The Reynolds School District — whose website is currently down — released a statement just after 9:30 a.m. saying Reynolds remains in lockdown, though law enforcement said shortly after they would be slowly busing students to the Fred Meyer parking lot.
The students will be systemically marked off a list and handed over to parents or guardians. The evacuation could take two hours.
About 2,800 students attend the school, one of the largest in the state. Wednesday was supposed to be the final day of the school year.