(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.
\"At the end he said: 'This is not a drill,'\" Infante told CNN.
A teacher also suffered non-life-threatening injuries.
The suspected gunman is also dead, the Multnomah County Sheriff's Office said. It was not immediately clear how he died.
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.
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.
CNN first learned of the shooting through reports on Twitter.
\"My thoughts and prayers are with students, staff and the Reynolds High School community during this difficult time,\" said Oregon Gov. John Kitzhaber.
CNN's Mariano Castillo, Steve Almasy and Stefan Simon contributed to this report.