Transcript for Cops Kill Hofstra Student in Home Invasion Standoff

Now, to a home invasion nightmare at hofstra university. Police trying to save a student who had a gun pointed at her head by masked intruder. An officer, making a split-second decision and firing, accidentally killing the young student. Abc's gio benitez has that story. Reporter: By l accounts it was a dramatic and chaotic scene. One officer's inside with the hostages and the gunman. Reporter: Police opening fire on the man who they say was holding andrea rebello hostage, killing her in the process. Everybody stay off the air. Reporter: Late sunday, students at this major private university, wore white ribbons to remember the 21-year-old junior at a graduation marred by grief. Please join us in a moment of silence in her memory. Reporter: And this morning, police say they are carrying out a full investigation of the shooting, which began early friday, when rebello, her twin sister and two friends returned to their offcampus house from a bar. The front door was left wide-open. Reporter: Around 2:00 a.M., police say this man, dalton smith, an ex-con on parole, stormed inside, wearing a ski mask, reportedly brandishing a semiautomatic gun. He forced everyone upstairs at gun point. Reporter: The students offered their computers and cash. When smith wasn't satisfied, he sent one woman out to the atm. She called 911 instead. When police arrived, they found smith holding rebello, a headlock, allegedly using her as a human shield. He's backing out towards that back door. He's got the gun to our victim's head. Eventually, points his gun at the police officer. Reporter: Police say that officer fired eight times. Seven shots hit smith. But one killed rebello. As her twin sister reportedly listened in horror outside. You can't imagine that anything will ever happen to a student. Nobody deserves a fate like this. Reporter: This morning, rebello's boyfriend gave abc news a statement. He said, quote, she was perfect. She had a beautiful face. But that was nothingompared to the beauty she had in her heart. Meanwhile, the officer who fired that gun has now taken sick

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