SHAWNEE, Okla. – A mother and her boyfriend were arrested last week in connection to the death of a toddler who reportedly had swelling and bleeding on the brain, according to CBS affiliate KWTV.

Authorities identified the suspects as 21-year-old Whitney Breslin and 22-year-old Cashe Graham. Both are being held in the Pottawatomie County Jail pending first-degree murder charges, according to the jail's website.

According to police, Graham called 911 to say that 2-year-old Jamie Murdock was having trouble breathing. When emergency responders arrived at the home they found the girl unresponsive. She was treated at two hospitals, but died on Saturday.

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Police reportedly say the doctors pointed out a bruise on the toddler's face and bruising on her abdomen.

"According to [the suspects'] story, they went in to check on her and she was having trouble breathing," said Detective Jason Crouch of the Shawnee Police Department. Crouch says the couple denies knowing how the little girl sustained the injuries.

Whitney Breslin and Cashe Graham KWTV

But KWTV reports that doctors said her symptoms would've appeared within an hour of the injury.

"It just tells us there is a real small window as to who is responsible for this," Crouch said. "And according to mom and the boyfriend's story, they were the only two there."

And according to the station, police say this was Murdock's third trip to the emergency room in a month.

She had two recent head injuries that the couple allegedly blamed on accidents.

"Mom and boyfriend said that she is a sleep walker and that she falls down, but the doctors don't think that's the case," said Crouch.

A police report notes the toddler died from a "non-accidental abusive head trauma."

The report also indicates the two-year-old tested positive for marijuana. Police say the amounts indicate she would've ingested the drugs, not secondhand smoke.

The couple admitted to keeping marijuana in their home, according to a police report.

"[The couple] understand how it looks, but still say they didn't do it," Crouch told the station, "but can't give me any other explanation as to what happened."