MILWAUKIE, Ore. - A 15-year-old Texas girl found dead in a Milwaukie apartment on Saturday died of strangulation, the state medical examiner's office has confirmed.

According to the Clackamas County Sheriff's Office, the teen, Dani Countryman of Kaufman, Texas, was visiting her older sister, 20-year-old Ashley Countryman of Oregon City, and both attended a party at the Balboa Apartment Complex at 2717 S.E. Courtney Road in Milwaukie on Friday night.

It was Dani Countryman's last night in the Portland area before she was due to return home.

According to Ashley Countryman, (pictured at right with her mother) the last time she saw her sister alive was about 5 a.m. Saturday. Dani was outside the apartments smoking a cigarette.

"She came outside with the biggest smile on her face just like Dani always was," she said.

Dani was watching movies with a group of teens in the apartment. They all seemed like nice kids, and Ashley Countryman (pictured at right) felt there was no reason to worry.

"She was just gonna go inside and lay down, watch the movie,” she said. “Everybody was going to bed."

But about three hours later, she went to check on Dani in apartment No. 21 and found her on a mattress in a back bedroom with a blanket over her upper body.

"I thought she was sleeping, and when I pulled the blanket over her head, she wasn't sleeping," she said. "She was cold. She was very cold."

There were bruises on her sister's neck and face.

"My sister is a fighter, and she's a very tough person," Ashley Countryman said. "And whoever did this to her definitely has marks on their body."

There were two other people sleeping in the same room just two feet from Dani, she said.

Ashley Countryman believes her sister was killed outside and then carried back into the apartment. Otherwise, someone would have heard the attack, she said.

"Whoever did this had the (guts) to carry her body back in there, pull a blanket off somebody and pull it over her face and lay her in there with everybody inside," Ashley Countryman said.

Authorities had not identified any suspects as of Monday.

Clackamas County investigators began investigating the death as a homicide on Saturday. They were interviewing a large number of people in connection with the death all weekend, the sheriff's office said.

Deputies have said the complex is a problem spot for them. In July alone, they have received various complaints about drinking, fighting and loud parties.