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The deaths of a teen girl and her infant sister in Ontario are now being investigated as a homicide, officials said Tuesday night.

The girls were found dead inside their home while their mother was found unresponsive, investigators said, but numerous questions remain.

The girls’ father called 911 around 4 p.m. to report unknown trouble at the residence on the 500 block of East Tam O’Shanter Street, said Ontario Police Sgt. Bill Russell.

The deaths of the 14-year-old and 4-month-old were initially being investigated as suspicious, but shortly before 11 p.m., police said they believe the two were killed.

The unresponsive mother was hospitalized with “slight injuries” and later listed in stable condition, Russell said. The sergeant did not describe the nature of the woman’s injuries and said he wasn’t sure how they were sustained.

“Because she was unresponsive, I don’t know if that was an internal thing or something that was done externally,” he said.

Police wouldn’t say if there was any sign of a struggle, but Russell did say there weren’t any noticeable noxious fumes in the air.

“This is highly unusual,” the sergeant said. “I’ve been on the job for 20 years. This is not a call you get very often.”

Coroner’s officials were at the scene Tuesday evening working to determine the children’s cause of death.

Police noted that there was no further threat to the neighborhood.

The father was taken to the police station for questioning but had not been placed under arrest, according to Russell.

Detectives were working to determine whether he was home when the deaths occurred or whether he came back to find his family members not breathing, Russell said.

“Right now, until we have an idea of if this was accidental or there was an intentional act, we can’t determine if anybody’s a suspect,” he said.

A man at the scene who identified himself as a cousin of the family’s said he was shocked by the news.

“I don’t think (there were) any problems,” Trac Pham told KTLA.

The investigation remained active Tuesday night, and no further details were available.

KTLA’s Nidia Becerra contributed to this report.

Ontario father finds 2 children home dead & wife unconscious. @OntarioPD says they don’t know if it was accidental or intentional. Police briefing us now: @ktla pic.twitter.com/x2Sqd4P1Uw — JENN MCGRAW (@NewsMcGraw) August 21, 2019

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