By Marty Graham

(Reuters) - A 15-year-old California girl has been arrested on accusations of using Instagram to anonymously threaten her San Diego-area high school, which was closed for two days this week as a result, police said on Wednesday.

Carlsbad High School, about 20 miles north of San Diego, closed mid-morning on Monday and sent students home after learning of the anonymous threat, Carlsbad School District spokesman Rick Grove said.

The school remained closed on Tuesday and re-opened on Wednesday after the girl was identified and arrested. The closure affected about 2,600 students and their families, Grove said.

Local media reported that the anonymous threat posted on social media read: “I am going to shoot up the school today at 11:30 or if all else fails Tuesday at 10:20.”

The school first learned of the threat just after the start of the school day, and had students shelter in place for an hour before sending them home, Grove said.

The Carlsbad police, working with a regional cybercrime team and the Department of Justice, on Tuesday identified the Carlsbad High School student they say posted the threat.

She was arrested without incident at her home,” police spokeswoman Jodee Sasway said. The girl was later released to parental custody, Sasway said.

Police declined to say why the girl may have made the threat.

(Editing by Cynthia Johnston and Eric Walsh)