Lance Corporal Sara Castro Mata was the youngest of three children and a graduate of Liberty High School in Brentwood, where she was on the honor roll.
Military officials have identified the gunman as Sergeant Eusebio Lopez of Pacifica. They say he killed Castro Mata and 23-year-old Corporal Jacob Wooley last Thursday night at the Marine Corps Base Quantico before he turned the gun on himself.
Our cameras stopped by the Lopez house Sunday, but family members did not wish to speak to us.
In a statement, Lopez's 81-year-old great grandfather said the Marines "told us they were investigating more, and they'd let us know. He wasn't the type to do stuff like that."
Investigators say Lopez was a teacher at a school that tests Marines who want to become officers and his specialty was machine-gunner. Castro Mata was a warehouse clerk for the school. And Wooley, of Guntown, Miss., was a field radio operator and also worked with the school.
The Marines have not described the relationship between the three or released a motive for the shooting.
(The Associated Press contributed to this report)