Spokane Fire Chief Brian Schaeffer confirmed the casualties at a news conference later Wednesday.
She said at least one person was shot but is expected to survive. Fire officials say that the threat "has been eliminated". Trauma surgeon Mike Moore said that the three children, who are all in their mid-teens, were doing great and are stable.
It is unclear if there are more victims or whether the shots were fired on school property.
Identities of the suspect and victims have not been revealed by authorities. The notes indicated that the troubled student meant to "do something stupid", the student being interviewed told reporters.
"He proceeded to take his weapons out".
A student says the gunman is a classmate.
"He was handing out notes to his friends and it said that he was going to do something stupid that might either get him killed or in jail", said student Michael Harper.
Schaeffer said one student was killed at the school.
According to the Gun Violence website, 253 mass shootings - defined as four or more people shot in a single incident - have been reported in the United States so far this year, including Wednesday's shooting in Washington state. "One will be requiring some surgery later this afternoon". The hospital will not release the injuries of the children until they are granted permission from their families.
Classes had just begun around 10 a.m. - a "late start" day at the school - when the shooting began. The two-lane road into town was clogged as people sped to the school.
The caption of the tweet, which was in reply to a tweet about the situation from Spokane News, read, "At Freeman elementary now. I've never heard him like that", Moser told The Spokesman-Review newspaper. "You never think about something happening like this at a small school".
All other schools in the area are on lockdown too.
#Frreman School Shooting-MiddleSchool & Elementary on lock down.
"These are senseless and tragic events that really don't need to happen and I don't understand them", he said.
Regardless she stressed for others she knew, including a collaborator who presently couldn't seem to get notification from her child, a sophomore at Freeman. She said the gunman wielded a pistol and a rifle, and had a "completely passive" look on his face when he opened fire.