Shocked ... The school serves at-risk students who were expelled or dropped out of other schools. (AP Photo/Greg Wahl-Stephens) Source: AP

The victims are students at Rosemary Anderson High School in the northwestern city of Portland or in related job training programs, police Sgt Pete Simpson said. A 16-year-old girl was in critical condition, and two males - ages 17 and 20 - were in serious condition, police said. A fourth person - a 19-year-old woman - was grazed by a bullet but not hospitalised.

The three were conscious and breathing when they were taken to the hospital, he said.

The shooting was reported after midday and happened at a corner outside the school, Simpson said.

At the scene ... Students ran back into the school and called for help when the gunshots were fired. (AP Photo/The Register-Guard, Bruce Ely) Source: AP

The assailant fled, and the students ran back to the school for help, he said.

Officers don’t know whether the shooter was alone or with others, but Simpson says preliminary information suggests it is gang-related.

Rosemary Anderson High School is an alternative school that serves at-risk students who were expelled or dropped out, are homeless or single parents.

The area has been troubled by gang-related violence for years and had a spike this year. (AP Photo/Greg Wahl-Stephens) Source: AP

According to the school’s website, 190 students annually are enrolled there.

Parents have been summoned to the school, and a nearby high school and community college were put on lockdown.

Portland has been troubled by gang-related violence for years and had a spike this year.

At midyear, the Police Bureau reported that officers had responded to 52 gang-related violent crimes, up from 35 through the same period last year, The Oregonian reported.

On the run ... Police are still trying to track down the shooter. (AP Photo/Greg Wahl-Stephens) Source: AP