EVERETT — The 15-year-old girl accused of stabbing two fellow students at Snohomish High School on Monday had planned since the weekend to attack someone, according to Snohomish County prosecutors.

The girl was ordered held for investigation for first-degree attempted murder, first-degree assault and second-degree assault during a juvenile court hearing Tuesday morning in Everett. Judge Michael Downes set bail at $1 million, saying she poses an "extreme risk" to the community.

In an affidavit of probable cause that outlines the case against the Snohomish High sophomore, prosecutors said she has no previous arrests, but has an extensive history of mental illness. They allege she had planned since Saturday to attack someone, although the reason is unclear.

"The suspect offered no reasonable explanation for her actions," a Snohomish detective wrote in the affidavit.

The Times generally does not name juvenile suspects unless they're facing criminal charges in adult court. The girl will face a hearing to determine whether she will be charged as an adult.

The girl's parents were in court on Tuesday, and her mother burst into tears when she saw her daughter. They declined to speak to the media.

According to the affidavit, the girl armed herself with an 8-inch butcher knife and a second knife before going to school Monday morning. She went into a girls' restroom on campus and waited in a stall for others to leave before attacking a freshman girl around 7:25 a.m..

During the attack, a second freshman girl came to the victim's defense and was also stabbed.

The victims were stabbed near an auxiliary gym on the campus, Snohomish Police Chief John Turner said. Police and paramedics rushed to the school, where the attack prompted a 90-minute lockdown.

The injured girls were taken to Providence Regional Medical Center Everett, where one was listed in serious condition Tuesday morning after undergoing six hours of surgery Monday for 20 to 25 stab wounds. According to the affidavit, her heart and lungs were pierced during the attack

The second victim suffered less severe injuries and was treated and released. She was resting at home Monday night, according to her stepmother.

The stepmother praised "the hospital, the school district and the cops. They've been wonderful." But she declined to say more before the suspect's first scheduled court appearance Tuesday in juvenile court.

The stabbings prompted the school of about 1,800 students to be placed on lockdown from about 7:30 a.m. until just after 9, district spokeswoman Kristin Foley said.

Although the school day resumed, many parents went to the school to pick up their children as news of the attack was reported by the news media and social-networking websites.

Turner said the suspect and victims did not know each other. He said he's heard a slew of theories and rumors about the motive for the attack, but that most aren't true.

"There are a lot of false stories out there," he said.

The two victims have been friends since they were young girls, Turner said.

They were described by fellow students as "really nice," "kind" and "caring."

The girls were "not the kind to get in fights and not the kind to get stabbed, " said Cyrena Sandoval, 14.

The suspect also is a nice person, according to Olivia Juel, 15.

"She is really quiet, but she is nice and actually very funny," Juel said.

Other students said they were shocked by the attack.

"It's scary that it happened at school," said a 14-year-old boy who did not want to be named.

In Tuesday, Snohomish police were at the school to reassure students that they were safe, police said.

Christine Clarridge: 206-464-8983 or cclarridge@seattletimes.com

Seattle Times staff reporter Jennifer Sullivan and researcher Miyoko Wolf contributed

to this report.