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IMPERIAL BEACH, Calif. – Another instructor at Mar Vista High School was accused of sex crimes with a minor - making him the third staffer to be arrested in a month, authorities confirmed Wednesday.

The San Diego County District Attorney's Office said 20-year-old Alejandro Rodriguez was arrested on March 9. The Mar Vista High employee was arraigned two days later and charged with three counts of oral copulation and one count of sodomy of a person under 18 years old.

Rodriguez was an in-school tutor, according to school officials.

It was unknown if the underage victim was a student at the school.

Edward Aaron Mendoza, who worked as a walk-on assistant football coach at the Imperial Beach campus last fall, was taken into custody Tuesday afternoon outside his current workplace in Chula Vista, according to sheriff's officials.

Mendoza, 27, is suspected of engaging in sex acts with the ninth-grade girl at the Elm Avenue school for roughly half a year.

The alleged crimes were reported after the student's father discovered that she was having "extensive phone conversations with Mendoza and was having an inappropriate relationship with him,'' Lt. Dan Brislin said.

"Detectives discovered Mendoza had fostered an ongoing relationship with the victim and engaged in sexual activity with her at school over a period of about six months,'' the lieutenant said.

Mendoza was booked into county jail on suspicion of multiple child- molestation counts.

The suspect has not worked at the school since December, when the campus football season ended, said Manny Rubio, spokesman for Sweetwater Union High School District.

On Saturday, detectives arrested Mar Vista ROTC instructor Martin Albert Gallegos, 48, for allegedly having sex with a 12th-grade female student on multiple occasions. Gallegos, a substitute teacher, was subsequently fired.