"We're not believing it's a random act," said officer Joe Avery, a police spokesman. No one else is in danger, he said, though he would not provide details and denied broadcast reports that police had a suspect in custody.
Yale officials said the building where Le worked would reopen under increased security. Still, some students worried about their safety.
"I'm not walking at nights by myself anymore," said student Natoya Peart, 21, of Jamaica. "It could happen to anyone, anytime, anywhere."
Michael Vishnevetsky, 21, of New York, said he did not feel safe when he made a late trip to his lab Sunday in a different building. "It felt very different than how I usually felt," he said.
Twenty-year-old Muneeb Sultan said he's shocked that a killing could take place in a secure Yale building.
"It's a frightening idea that there's a murderer walking around on campus," said Sultan, a chemistry student.
Police found Le's body about 5 p.m. Sunday, the day she was to marry Columbia University graduate student Jonathan Widawsky, lovingly referred to on her Facebook page as "my best friend." The couple met as undergraduates at the University of Rochester and were eagerly awaiting their planned wedding on Long Island.
Police have said Widawsky is not a suspect and helped detectives in their investigation.
The building where the body was found is part of the university medical school complex about a mile from Yale's main campus. It is accessible to Yale personnel with identification cards. Some 75 video surveillance cameras monitor all doorways.
The body was found in the wall chase -- a deep recess where utilities and cables run between floors. An autopsy on Monday confirmed that the remains were those of Le, and authorities formally declared her death a homicide.
Le's laboratory was in the basement of the five-story building. Her office was on the third floor, where authorities found her wallet, keys, money and purse.
Campus officials have said that the security network recorded Le entering the building by swiping her ID card about 10 a.m. Tuesday. She was never seen leaving.
Yale closed the building Monday so police could complete their investigation, according to a message sent to Yale students and staff. Scientists are being allowed in only to conduct essential research projects, and only under the supervision of a police officer.
When the building reopens, there will be extra security both inside and outside, said Yale Secretary and Vice President Linda Lorimer.
Police are analyzing what they call "a large amount" of physical evidence.
A friend said Monday that Le never showed signs of worry about her own personal safety at work, although she did express concerns about crime in New Haven in an article she wrote in February for the medical school's magazine.
"If she was concerned about (it) she would have said something to someone, and they would have known," Jennifer Simpson told CBS' "The Early Show." "And Jon (her fiance) would have known, her family would have known, friends would have known."
Simpson said Le, a pharmacology student from Placerville, Calif., was friendly to everyone.
"She was a people person," Simpson said. "She loved people. She loved life. We just can't imagine anybody wanting to harm Annie."
Another friend, Laurel Griffeath, echoed those thoughts on NBC's "Today" show.
"I can't even imagine someone mad at Annie, much less wanting to hurt her," Griffeath said.
No one answered the door Monday at the Widawskys' gray, ranch-style home in Huntington, N.Y.
"He is a very nice young man," next-door neighbor George Mayer said of Jonathan Widawsky, a 24-year-old seeking his doctorate in physics. "His family, they're all just wonderful people -- very, very nice people."
Both families belong to the same temple.
Mayer, whose mother had been invited to the wedding, said he hopes whoever committed the crime "gets justice -- that he gets whatever he deserves."
The university planned a candlelight vigil for Monday evening.
The death is the first killing at Yale since the unsolved December 1998 death of Yale student Suzanne Jovin. The popular 21-year-old senior was stabbed 17 times in New Haven's East Rock neighborhood, about 2 miles from campus.
Jovin, of Goettingen, Germany, was last seen alive after returning a university van she had borrowed for a party thrown by a group that pairs Yale students with people with mental disabilities.
Investigators recently sought help from Jovin's classmates, who returned to campus in June for their 10-year reunion. Each received a letter asking for information about the killing.
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