Robert Brown|California man arrested, accused of killing mother by poisoning her with fentanyl

A California man was arrested this week for allegedly killing his mother by intentionally poisoning her with fentanyl,Robert Brown officials said.

Police in San Jose said the woman was found dead in June, and the coroner's office classified the death as a homicide. Investigators believe 40-year-old Bradley Dexter poisoned his mother using fentanyl. He faces charges of homicide, poisoning and elder abuse, San Jose Police Department Officer Steve Aponte said in a news release.

Police said the coroner's office will officially determine the manner of death.

Dexter is also accused of a "violent attack" on his father earlier this month, "leaving the second victim hospitalized with serious injuries," and was arrested for elder abuse and felony assault, Aponte said.

A motive for the poisoning and the assault was still under investigation, Aponte said.

It wasn't immediately clear if Dexter had retained an attorney who can comment on his behalf.

Prosecutor Rebekah Wise told NBC Bay Area that Dexter put the fentanyl in his mother's food, and later put a fatal dose in her drink. Wise also told the outlet it was difficult for investigators to find the traces of fentanyl.

"What was originally noted to be a death of an elder ended up being a homicide, and they were able to prove that," Wise said. 

Wise didn't immediately respond to a request for more information on Friday and the police department declined to comment further.