PORTLAND, Ore. (AP) — Someone opened fire on a group of young people outside an alternative high school Friday, sending three people to the hospital in what is believed to be a gang-related attack, Portland police said.

The victims are students at Rosemary Anderson High School 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 hospitalized.

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

Witnesses told police there may have been a dispute outside the school, but police said they didn't know who was involved.

"We don't know what led up to the shooting," Simpson said. "There was some kind of dispute."

The assailant and two other people fled, and the wounded students went to the school for help, he said.

A nearby high school and community college were put on lockdown.

Preliminary information suggests the shooter has gang ties, Simpson said. The Police Bureau's gang unit was deployed in the investigation.

Sierra Smith, a 17-year-old student, told The Oregonian she saw one of the male victims being helped by a teacher inside the school.

"He was laying on the ground. He had blood coming out of his stomach," she said. "It was scary."

Another student, Oliviann Danley, 16, told the newspaper she saw a boy run into the school and yell, "Oh my god, did I just get shot?"

Rosemary Anderson High School serves at-risk students who were expelled or dropped out, or who are homeless or single parents. According to the school's website, 190 students annually are enrolled at the north Portland location. The school also has a second location in Gresham.

Gang violence in Portland isn't a new phenomenon. Some of the violence occurs between rival gangs, but bystanders have also been hurt.

"We've made a lot of progress in addressing the gang problem, but we haven't eradicated it," Mayor Charlie Hales said after shooting Friday. "Today's really a sad reminder that it's still with us."

Portland police have said they saw a spike in gang crime this summer and have complained they don't have adequate resources to address the problem. Recent violence includes a man killed in a drive-by-shooting in June and another man killed in a separate shooting. A 5-year-old boy also was shot in the leg while playing at an apartment complex.

A Multnomah County report on gang activity released in June said crime in the county that includes Portland actually decreased from 2005 to 2012. As inner-city Portland gentrifies, the report said, criminal activity is shifting from northern neighborhoods to areas farther east, including the city of Gresham.

The report identified at least 133 active gangs in the county.

Dani Gonzales, 64, has lived in the neighborhood of Friday's shooting for 25 years and said it's generally safe but there has always been some gang activity.

"Kids just get silly and get crazy ideas. I don't know what goes on in their heads," Gonzales said.

There was another school shooting in the Portland area in June, but it was not gang-related. A freshman killed another boy in a locker room and a bullet grazed a teacher before the shooter went into a bathroom and died from a self-inflicted gunshot, police said.

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Associated Press writers Terrence Petty, Gosia Wozniacka and Tim Fought contributed to this report.