Police have said O'Sullivan was assisting a woman removing belongings from a home in the north Sacramento neighborhood when the suspect shot her.
"It's the drugs, it's always been the drugs", Orlando Ramos said.
Orlando Ramos said she was Adel Ramos' girlfriend.
The family could not immediately be reached for comment.
Prior to working for the Sacramento Police Department, she was a former police explorer serving both the Martinez and Pleasant Hill Police Departments.
Peletta said O'Sullivan was with another officer when she was shot just before 6 p.m. Wednesday.
Gov. Gavin Newsom said in a statement that O'Sullivan "represented the best of what we hope to be as human beings in her selfless service to the community and readiness to help those in need". She was part of the first class of graduates of Sacramento State's Law Enforcement Candidate Scholars program in 2017 and went on to the Sacramento Police Academy.
Her professors and university leaders recall the 26-year-old O'Sullivan as a leader among her peers and someone with tenacity. A half-hour later the first shots were fired, and O'Sullivan was hit, authorities said.
They described her as someone who took on the physical training aspects of the program with vigor, even though she was small in stature.
O'Sullivan had only been a police officer for six months when she was killed, after graduating from the force's police academy in December.
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'Our officers maintained cover in safe positions until we were able to get an armored vehicle in the area, ' said Sgt. Vance Chandler.
City manager Howard Chan said the officer died "selflessly coming to the aid of others".
The suspect in the shooting has not been publicly identified, but he is in custody.
A law enforcement expert says it's "unacceptable" that it took Sacramento police officers almost an hour to reach a colleague who was wounded during a domestic violence call and later died.
Police said they would release more details about the incident when it is no longer active. Image via Facebook and Sacramento Police Bookings.It is with a broken heart that we have to share with all of you that earlier today we lost one of our own.
"We are devastated tonight", he said.
The department said detectives are gathering evidence at the home in a Sacramento neighborhood.
According to cops, officers got a call late Wednesday morning (around 11:43 a.m.) regarding a domestic disturbance involving a man and a woman.
O'Sullivan was pronounced dead at UC Davis Medical Center within hours, Deputy Police Chief Dave Peletta told reporters during a news conference at around 12:40 a.m. local time. But she added that she "wouldn't want young girls to walk away thinking that this happened because she was a woman".
Police say the suspect who has not been identified fired his rifle on and off during an 8-hour standoff before he surrendered. She died at a hospital.