She has been described by friends as a "brilliant" student, eager to pursue a research career in medical anthropology. He was responsible for cleaning mouse cages in a university lab.
Authorities have not yet pinned down a motive for the killing -- in which Clark was accused of murder after Le's body was found hidden in a laboratory wall on Sept. 13, the day she was to be married.
New Haven, Conn., Police Chief James Lewis told The Associated Press officials may never know why she was killed.
"The only person who knows the motive is the suspect," Lewis said. "It's true in many cases. You never know absolutely unless the person confesses, and in this case it's too early to tell."
Brad Garrett, a former FBI agent and an ABC News consultant, said the fact that Le was found strangled may hint at a motive in the case.
"People who kill other people and strangle them is a very personal thing because they are actually looking at you as they are dying," Garrett told "Good Morning America Weekend." "Think about what anger and rage anyone would have to have."
If Clark committed the crime he is accused of, a criminologist told ABCNews.com his actions might be explained by what sociologists call "relative deprivation."
That psychological mind set can be triggered "when you are measuring your own self worth against others and you come out on the bottom," according to Northeastern University criminologist Jack Levin, who has been following press accounts of the case.
"[Ray Clark] worked in an Ivy League school where most of his co-workers were potentially successful and had advanced degrees and were looking forward to a fulfilling and happy life," said Levin. "He was cleaning cages."
Le's research on enzymes could have had involved breakthroughs in cancer, diabetes and muscular dystrophy. Clark's work, on the other hand, tidying floors and cages, was more janitorial in nature.
Ann Turner, executive director of the American Association for Laboratory Animal Science, told the Hartford Courant "there is a gulf" between technicians and the researchers they interact with, but that gap would not necessarily led to tension.
"If there is a culture of trust and a culture of respect, the researchers will respect the animal care technicians, and vice versa," Turner said.
Since his arrest Sept. 17, Clark has not entered a plea and neither he nor his lawyers have not spoken, but police say evidence is mounting against the suspect in what they are calling a case of workplace violence.
Some friends told ABCNews.com said they could not imagine Clark -- "a nice guy" -- committing a crime. Others who knew him, however, described him as a "control freak" who was competitive in sports, compulsive about his work habits and controlling in his romantic relationships.
One law enforcement official who talked to the AP on condition of anonymity said this week that Clark's co-workers said he was territorial about the mice whose cages he cleaned.
Need for Control Can Be Motive
The psychological need for control is often "underestimated as a motive for murder," according to Levin.
"When people think of motives for murder, they tend to think of money, jealousy, revenge, but they don't think of a killer's excessive need for power, dominance and control," he told ABCNews.com.
Le's body was found Sept. 13 -- the same day she was to be married. But police have ruled out romantic involvement between the accused killer and her victim.
But when police ruled out romantic intentions and Levin learned about reports that indicated Clark had a controlling personality and a low-level job, he said it could make sense theoretically.
Though no one knows what happened between the victim and her killer in that lab, "a controlling individual might decide to take things into his own hands and the wedding may have been the precipitant."
"All that did was reinforce the idea that his victim was going on to have a satisfying personal life and career, just another indicator for him that he was going nowhere," Levin said.
He added that sometimes acts of violence may be precipitated by the killer "trying to reestablish control over a situation that is out of control."
"Someone is fired and decides to seek sweet revenge through the barrel of a gun aimed at a boss or co-workers, or a husband-father is caught in a nasty separation or divorce," Levin said.
Levin, author of "Serial Killers and Sadistic Murderers Up Close and Personal," said mass murderers, as well as "work place avengers," are often so-called "control freaks."
Software Engineer Kills Co-Workers
In a case in Massachusetts in 2000, Michael McDermott, a 42-year-old software engineer, used a semiautomatic rifle and 12-gauge shotgun that were hidden under his desk to kill seven of his co-workers at Edgewater Technology Inc.
Prosecutors have said he was angry over a government demand that the company withdraw back taxes from his paychecks.
"He blamed these people for his personal miseries even though it was the IRS that was garnishing his wages," said Levin. "It was his way of gaining control. There are a lot of killers who feel powerless."
Levin suggested that Clark, from a middle-class Connecticut family and dressed in khakis and a polo shirt when he was arrested, might have felt a failure not to live up to his parents expectations.
"We think of control as obsessive compulsion, but another control is regaining a sense of power by squeezing the life out of a happy, successful co-worker," said Levin.
Controlling Personality Could Point to Rage, Says Expert
The strangulation death also suggested that the killer -- whether Clark or someone else -- likely had a volatile personality, as well as control issues, he said.
"An up-close and personal murder requires a tremendous amount of anger," according to Levin.
Controlling men often display their anger in their relationships with women, according to Levin.
"They want to make their victims suffer or to play God by deciding who lives and who dies," he said. "There are certain men who treat their women like possessions. They may be very jealous, possessive and feel if I can't have her, nobody can."
Neighbors of Clark told ABC News' "Good Morning America" that he had a "menacing" relationship with his fiancee, Jennifer Hromadka.
Clark worked alongside Hromadka at the Yale lab, and also lived with her at a nearby apartment complex known as Wharfside Commons.
Annmarie Goodwin, a woman who was Clark's neighbor in Branford, said Clark was "very controlling of his girlfriend. He wouldn't let her talk to me or anything."
Hromadka described Clark as "a bit naive ... but a good guy."
Clark's teammates, who were playing softball with him before his arrest, told ABC that he seemed completely normal that day, as his parents and fiancee sat in the stands.
"I still don't believe it," said Luz Viera. "I'm still thinking maybe tomorrow they'll say release him, it's not him."
"He's a good kid, no identification in his behavior the whole season or last Sunday that he was capable of doing a heinous crime like this," said Richard Santana.
Meanwhile, Annie Le's grief-stricken family has made private funeral preparations for their daughter, as the body has been flown back to her native California.
ABC News' Emily Friedman and Stephanie Sy contributed to this report.