SAN FRANCISCO (AP) — A teenage gunman returned to class Tuesday after shooting a fellow student multiple times just outside a Northern California high school, an attack that prompted an hourslong lockdown of more than 10,000 students and staffers in a complex with three schools, authorities said.

Officers arrested the 17-year-old shooter on suspicion of attempted murder in a classroom at Ridgway High School in Santa Rosa, north of San Francisco, after a less than two-hour manhunt.

Police Capt. John Cregan said officers were still looking for the shooter's handgun, which he put into a backpack and handed off to someone else before going to class.

Police do not yet know if the person who took the backpack and drove away knew what was inside. Authorities said the two students argued before the shooting and believe the violence was isolated.

The victim, a 16-year-old boy, was in stable condition after being shot twice in the upper body on the sidewalk outside the high school.

Authorities initially said the shooter had an accomplice, but Cregan said the person who had been detained was released. Detectives were working to determine if the shooting was gang-related, he said.

Witnesses told police that the shooter fired three to four rounds before running onto the campus after classes began, authorities said.

Parents described the panic of getting text messages from their kids and not knowing if they were safe. One mother, Cherie Posluszny, said her 17-year-old daughter texted just after 9:40 a.m.

"Someone got shot," the first message said, followed up a minute later by: "I love you."

"Someone is at Ridgway," read the next message. Then her daughter's phone stopped working. She found a friend's phone and her mother asked if she was OK.

"Not really. But I will be OK," her daughter responded. The girl told her mother that she spent hours lying on the cold floor hiding behind a teacher's desk with other students.

Posluszny said she worried about how this "new normal" was affecting children.

"This is becoming routine. It's part of the culture now. These kids are also victims," she said as she raced to the school.

Police said the shooting was originally reported on the Ridgway High campus just before 9 a.m. but now say it was on the sidewalk just outside the school. It is part of a sprawling complex that also includes Santa Rosa High School, Santa Rosa Junior College, school district offices and athletic fields.

The complex with thousands of students, teachers, administrators and others was locked down as a SWAT team searched for the gunman, Cregan said.

TV footage from a helicopter showed a boy or man being led away in handcuffs while students were taken out of a building and searched.

Santa Rosa police, who are getting help from the FBI in the investigation, lifted the lockdown around 11:30 a.m. following a "thorough sweep" of the schools, saying on Twitter that "there is no threat."

School district officials referred questions to police but tweeted that "students are safe."

Ridgway High, about 55 miles (89 kilometers) north of San Francisco, is an alternative continuation school for Santa Rosa High.

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Dazio reported from Los Angeles. Associated Press writer Jocelyn Gecker contributed from San Francisco.

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This story has been updated with new information from police that the shooting happened near the school, not on campus, and that the victim was 16, not 17.