Three men suspected in a gang-related shooting this month outside an Oregon high school for homeless and other vulnerable students are facing attempted murder and other charges, Portland police said on Monday.

Police said the Dec. 12 shooting, which left four people injured, erupted following a dispute outside Rosemary Anderson High School.

Sixteen-year-old Marquise D'Angelo Murphy, 18-year-old Marquel Diamond Dugas and 22-year-old Lonzo Murphy face attempted murder, assault, and four counts each of unlawful use of a weapon, among other charges, police said in a statement.

Portland Police Bureau spokesman Pete Simpson said the suspects are expected to be arraigned on Tuesday in Multnomah County Court. A grand jury was underway to consider the charges.

Three of those hurt have since been discharged from the hospital, police said on Monday. A fourth victim was grazed and did not go to the hospital.

Dugas and Lonzo Murphy, Marquise's brother, were arrested on Dec. 13. Marquise Murphy was arrested on Dec. 16. All are being held on probation violations pending the investigation, police said.

Police have said they believe the shooting was gang related but they have not released details as to what the dispute was about.

Rosemary Anderson High School serves "at-risk" students including some who are homeless or have been expelled from or dropped out of mainstream schools.

(Reporting by Shelby Sebens in Portland, Oregon; Editing by Eric M. Johnson and Jeremy Laurence)