MILLERSVILLE, Pa. » The boyfriend of a college freshman found dead in her dorm room was charged Monday with homicide after an autopsy found she had been severely beaten and strangled, during a struggle in which a prosecutor said she "fought for her life."

Police officers responding to a 911 call early Sunday from the boyfriend, Gregorio Orrostieta, 19, said they found him trying to administer CPR to Karlie Hall, 18, his own face, hands and jeans smeared with blood, his shirt ripped, his chest scratched and his forehead cut, a police affidavit said.

Authorities believe the CPR was "completely fake," and that Hall had already been dead for hours when Orrostieta called them to Millersville University, claiming the young woman had suffered a heart attack, said Lancaster County District Attorney Craig Stedman.

Questioned by police, Orrostieta said he shoved Hall, causing her to fall and hit her head on a chair, and then gave her a "back hand" to the face, the affidavit said. But he made no mention of choking her.

"He’s responsible and we’re going to hold him accountable," Stedman told a news conference to announce the homicide charge. He had been charged only with aggravated assault pending the autopsy.

"She fought for her life," the prosecutor said.

The dorm’s video surveillance confirmed that no one else entered or left the room during the time when a struggle was heard in the dorm room.

The couple, both from the Philadelphia suburbs, had been dating about 11 months.

Orrostieta, of Kennett Square, is not a Millersville student. He was being held without bail. The prosecutor said he did not have a lawyer on record, and no one answered the door at his residence Monday afternoon.

Orrostieta told police that he and Hall, 18, of Chadds Ford, had argued at a party Saturday night and she had hit him, the affidavit said. They made up before returning to the dorm at around 1:30 a.m. and began arguing again, the affidavit said. Stedman declined to say if there was a history of abuse.

Stedman said witnesses reported hearing yelling and the sound of furniture moving between 2 and 2:30 a.m., about the time authorities believe Hall was killed.

The dorm’s resident assistant knocked on the door, but no one answered and no further sound was heard, authorities said.

Police found Hall unresponsive when they arrived at Bard Hall after getting the 911 call at about 5 a.m., authorities said.

Asked about the resident assistant not calling police, Stedman said: "They’re not criminal investigators. Their responsibility is … to make sure there’s no more disturbances going on."

"It’s something that we’ll look at," he said while adding: "I don’t want in any way anybody taking out of this press conference today that anybody’s responsible for her death other than the defendant who’s charged."

University spokeswoman Janet Kacskos said the RA was awoken by other residents who heard the noise. After knocking and not getting any response, she figured they had "quieted down," Kacskos said.

In a note to students, Millersville President John Anderson called Hall’s death "unfathomable." "We’ve never had this happen. We’re a pretty bucolic, rural campus. Very safe," Kacskos said.

Hundreds of students attended an outdoor vigil for Hall Monday night at Millersville, an 8,000-student state-owned university. Standing in the rain and holding candles, many of them wept as they sang two hymns and campus minister Dwayne Netzler prayed, the Reading Eagle reported.

Friends said Hall was a finance major who always appeared happy and often spent her free time going to the gym or feeding ducks at a campus pond.

"I knew that the relationship wasn’t that great," said Hall’s friend, Trisha Faust, 19, of Emmaus. "It was on-again, off-again."

Molly Gaetano, 19, of Pittsburgh, who lived two doors down from Hall on the second floor of the three-story dormitory, said she last spoke to her Friday.

"She never talked bad about anyone. She was always smiling and cheerful," Gaetano said.

A memorial with flowers and cards was set up at Hall’s dorm room.

Hall and her twin sister, Kristin, graduated from Unionville High School last June and went to Millersville together, Principal Paula Massanari said. The girls also have an older sister.

Hall was a member of the school rugby club and gay-straight alliance, and she volunteered at an animal shelter, Massanari said. She was described in a college recommendation letter as a "hard-working" student, who was working a part-time job to help offset the cost of college.

"This has certainly hit our school community very hard," Massanari said. "We are devastated by the loss."

Associated Press writers Sean Carlin and Maryclaire Dale in Philadelphia contributed to this report.