The family of an autistic California teen who drowned during a school trip says it wants to know why the teaching assistant assigned to watch over him at all times "wasn't doing his job."

Erick Ortiz, 16, a sophomore at Garfield High School in Los Angeles, went to a local park Wednesday afternoon for an end-of-year celebration with his classmates and his teaching assistant, the family's attorney, Luis Carrillo, told ABC News. Erick was later pulled from the park's pool with his clothes on and pronounced dead at a local hospital.

"We want answers," Erick's sister, Leslie Ortiz, said during a press conference at Carrillo's office Thursday. "Where was the TA? Why wasn't he with him? Why wasn't he taking care of him? Why wasn't he doing his job?"

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Erick did not know how to swim, Carrillo said. He could not communicate verbally with anyone outside his family, and he'd been with a school-assigned teaching assistant since 2012.

The assistant's job was to watch Erick at all times because he'd wandered off and hurt himself in the past, resulting in scrapes on his legs, Carrillo said. In 2013, the school district attempted to eliminate the teaching assistant, according to Carrillo, but Erick's mother fought them to keep the position.

Claudia Herrera, Erick's mother, signed a permission slip for the park field trip, but said she was not told about the pool, Leslie Ortiz said during a press conference.

The family plans to file a wrongful death claim next week, Carrillo said.

"The gross, reckless negligence is that he [the teaching assistant] was not with Erick, and somehow Erick, because of his incapacities, wound up in the pool," Carrillo told ABC News.

The Los Angeles Unified School District said it will "continue to cooperate with the ongoing law enforcement investigation."

"The District extends heartfelt condolences to the family on its tragic loss," the district said in a statement. "Crisis counselors are on site to help the school community. We will have no further comment at this time."