(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.
An assistant principal told students to go into lockdown mode.
Student describes the Oregon shooting Police: Oregon school shooter dead Student's parent: We just panicked
Photos: Shooting at Oregon high school Photos: Shooting at Oregon high school
"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."
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 CNN affiliate 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.
About an hour after the shooting, Oregon State Police said the area was secure and the situation was contained.
Speaking in Washington, President Barack Obama said the nation should be ashamed of its inability to get tougher gun restrictions through Congress in the aftermath of mass shootings that he said have become commonplace in America.
"Our levels of gun violence are off the charts. There's no advanced, developed country on Earth that would put up with this," he said in response to a question about gun violence.
Most members of Congress are "terrified" of the National Rifle Association, the President said, adding that nothing will change until public opinion demands it.
"The country has to do some soul searching about this. This is becoming the norm, and we take it for granted, in ways that as a parent are terrifying to me," Obama said.
Copyright 2015 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.