They were quiet and studious, two 20-year-old premed students from Third World countries for whom the journey to Harvard had been long and difficult. It made sense that they were roommates, not murderer and victim.

The killing of Trang Phuong Ho, a Harvard junior stabbed 45 times by her classmate Sinedu Tadesse early Sunday morning, and Tadesse's suicide a few minutes later, have left a shocked Harvard community searching for explanations of how a friendship could have come to such a violent end.

The two deaths on the last day of the spring term cast a pall over the campus, where seniors usually would be preparing to celebrate next week's commencement.

Harvard President Neil Rudenstine, who visited the crime scene Sunday morning, wrote to the residents of Dunster House, the dormitory where Tadesse and Ho had shared a room for the last two years, saying he was "deeply saddened" by the tragedy.

"Everyone stands ready to be as helpful as possible, so I urge you to be in touch for any reason," Rudenstine wrote.

James Rowe, a Harvard vice president, said the school considered the attack a tragedy that, for now, defied explanation. He said the families were making funeral plans and that Harvard may hold a memorial service.

Prosecutors are investigating whether the crime was premeditated. In a bizarre twist, the Harvard Crimson, the student newspaper, reported receiving a photo of Tadesse last Tuesday accompanied by a note that said the woman soon would be the subject of a "juicy story." Crimson editors tossed the note away, and police had to fish it out of a trash bin.

It is known that sometime this spring, Ho decided not to room with Tadesse again and told her. A possible indication of Tadesse's state of mind was contained in a letter Tadesse sent to Ho last month, according to Ho's sister, Thao, a student at nearby Tufts University. "You'll always have a family to go to and I am going to have no one," the letter said, Thao said in an interview with the Boston Globe.

Ho's family lives in Medford, just a bus ride from Harvard, and she visited them almost every weekend. Tadesse had not seen her family since leaving Ethiopia three years ago, and acquaintances said she had few friends.

A graduate of the prestigious International Community School in Addis Ababa, where she was president of the student government, Tadesse was described by Maura McMillin, a teacher at the school, as a product of her culture, except that she was more outspoken than many young Ethiopian women. McMillin told the Crimson that Tadesse was attending Harvard on a full scholarship.

Ho fled her native Vietnam on a boat about 10 years ago and was the valedictorian at Boston Technical High School. A vice president of the Harvard Vietnamese Student Association, she tutored refugees in Cambridge. She also volunteered at a homeless shelter for women and worked in a lab at the Dana-Farber Cancer Institute in Boston. She had wanted to be a pediatrician.

Tadesse allegedly attacked Ho as she lay in her bed, lying head to foot with a visitor, 26-year-old Thao Nguyen. Nguyen, who survived the attack, fled to seek help. Ho apparently struggled with her attacker, according to the autopsy, which found cuts on Ho's hands. She was also stabbed on her face, neck, chest and legs. Tadesse then ran into a bathroom, rigged a noose in the shower stall and hanged herself. She was cut down and rushed to Cambridge Hospital, where she was declared dead.

The only eyewitness was Nguyen, who told the Boston Herald that she awoke Sunday morning to see Tadesse hacking her friend with a knife. "I think she planned it," Nguyen said, adding that Tadesse was "crazy looking" and seemed bent on killing Ho.

"I tried to get the knife," Nguyen said. "But I couldn't."

Nguyen said she was visiting Ho to help her pack her things. Nguyen said Ho and Tadesse had become estranged and that on Saturday she had heard Tadesse weeping.

Murders on the campus are rare, and officials could not recall a case of a student killing another.

"What's very telling is the fact that the victim was stabbed 45 times," said James Alan Fox, dean of the college of criminal justice at nearby Northeastern University. "That signifies anger and blame. If you just want someone dead, you don't have to do it 45 times."