An assistant principal at San Marino High School (search) was arrested for allegedly sending herself 39 threatening letters she claimed were sent by students.

Mary Andrea Mitchel (search), 41, pleaded not guilty on Tuesday to 123 charges and she was jailed after Superior Court Judge Barbara Lee Burke set bail at $480,000 bail.

Mitchel notified the San Marino Police Department about the threatening letters, which began arriving in March and continued through November, Deputy District Attorney Shelly Torrealba said. Some of the typewritten letters contained white powder, she said.

Mitchel offered seven names of students who might have sent the letters and the students were investigated but never arrested, Torrealba said.

The assistant principal was given escorts and surveillance, Torrealba said, and the Police Department appealed to the FBI, the South Pasadena Police Department and U.S. Postal Service for help in the investigation.

Mitchel eventually admitted she wrote the letters to get attention, the prosecutor said.

The San Marino Police Department expended 780 man-hours on the investigation worth about $33,000, which it will seek to recoup. Mitchel was also charged with threatening the high school's resource officer, who is a police employee.

Defense attorney Michael Mayock (search) said Mitchel and the resources officer Jim Henson were romantically involved.

"She was attempting to extricate herself from a situation she couldn't put up with. That was a cry for help," Mayock said.