(CNN) On the nights the father can sleep, he awakes crying.

The grief becomes even more overbearing during the day, when his only daughter stares back from photographs in the family home. Her presence lingers in every room. Sometimes, the father must close his eyes.

His daughter was an extraordinary student. Loved the arts. Played piano. Dreamed of becoming a sculptor. She devoured her mother's homemade dumplings and hot pots.

Mother and father scrimped and saved. Part of China's burgeoning middle class, they tucked away their life savings, more than $100,000, for her education. They were thrilled to send her to the United States for college; she hoped to repay them one day.

Tong Shao majored in chemical engineering at Iowa State University in Ames, a field that made her father happy because not many women are brave enough to enter the male-dominated profession.

Like so many of today's Chinese youth, Tong was the product of Beijing's one-child policy. She had grown up the only child in a coastal city in northeastern China. Her mother and father were sold on the idea of sending their daughter to a bucolic setting in rural America. In central Iowa, they believed she would be safe.

But last September, the 20-year-old college junior was found stuffed in the trunk of her car. Killed more than 6,500 miles from her home. Her body rotted in the heat for three weeks before the gruesome discovery.

"We've given all our love to our daughter," Chunsheng Shao says through an interpreter. "I feel my life is meaningless after losing her."

It's been more than six months since Tong's mother and father were notified of her death. They remained quiet in their sorrow and figured U.S and Chinese authorities would find their daughter's killer. They decided to speak up now, hoping it might force investigators to do more.

"We are miserable, as the killer is still at large," the father says.

The last person to see Tong alive, police records show, was her 23-year-old boyfriend, Xiangnan Li, a Chinese student studying business at the University of Iowa. Iowa authorities say they want to talk with Li. He and Tong stayed in a hotel together in early September. The same weekend, police say, Li bought a one-way plane ticket to China and vanished.

Tong's father weeps. "What has she done to deserve such a crime? Why?"

Tong Shao with her father in China. The killing has devastated him: "I can't stop thinking about this whole thing."

No extradition treaty complicates case

Iowa and China have an exceptionally strong relationship, one that dates back more than 30 years. Iowa and the province of Hebei formed a sister-state relationship to build trade relations, forge business ties and form global friendships.

A young Xi Jinping -- now the president of China -- traveled to Iowa in 1985 on an agricultural tour, getting a firsthand look at America's farming technology. When Xi visited the United States in February 2012 before becoming the leader of the world's most populous nation, he visited three places: Washington, California and Iowa.

At a private dinner in the town of Muscatine, Xi ate with friends he met on his trip decades before. "You were the first group of Americans I came into contact with," Xi told his Iowa friends . "To me, you are America."

Those strong ties can be seen on the campuses of Iowa's two flagship universities, Iowa State University in Ames and the University of Iowa in Iowa City. At both, 1 in every 10 students is Chinese.

The killing of Tong Shao has shaken the more than 5,000 Chinese students across the state. Many wonder whether authorities have done enough to seek justice.

Some ponder: What would happen if I went missing? Would anyone be held accountable?

There is little precedent for a case like this -- when a Chinese student is wanted for questioning in the killing of another Chinese student on U.S. soil.

There is no extradition treaty between the nations, and the likelihood of China handing Li over for questioning -- if he can be found -- is slim, according to legal scholars.

"China generally does not in any case extradite Chinese citizens, so the most likely outcome were this person to be found would be prosecution within China," says Ben Liebman, director of the Center for Chinese Legal Studies at Columbia Law School.

"China will prosecute people within China for crimes they commit against citizens overseas."

A murder case in 2010 in which a Chinese citizen killed a taxi driver in New Zealand and fled to Shanghai was eventually tried in China, over the objections of Auckland officials who wanted the suspect tried in their country. The man was sentenced to 15 years in prison. Media reports at the time said it was the first time a killing in New Zealand was tried outside its borders.

Johnson County Attorney Janet Lyness is leading the prosecution in Iowa City. When quizzed about the case, she was guarded. She said Li is wanted for questioning and "certainly a person of interest" because he "would have information that would be helpful to the investigation."

Police records show Xiangnan Li, 23, stayed with Tong the weekend she disappeared. He then left for China.

"When somebody who may be a witness is not in this country, obviously that adds to the complexity," Lyness says.

The FBI would not comment on the case, saying only that agents are assisting in the investigation. China's Ministry of Public Security has not responded to CNN's questions about what it is doing in the hunt for Li.

Tong's family is left to wonder whether justice will ever prevail.

Her father issued a plea in late January for U.S. officials to share everything they know with Chinese authorities. He puts the blame for the killing squarely on his daughter's boyfriend.

"We now plead with you, the U.S. authorities, to issue an arrest warrant ... and share the evidence you have gathered with the Chinese authorities," he wrote. "This honorable act could very well save the life of other innocent and vulnerable victims, and will most assuredly allow the soul of our precious daughter to be comforted and rest in peace."

Her father is aided by hundreds of Chinese who have taken to WeChat, an instant messaging tool and social media platform, and Weibo, Beijing's version of Twitter. They post photographs of Xiangnan Li and ask for anyone who has seen him to turn him in. They use the hashtag #FindLi.

"Everybody please #FindLi," one Chinese graduate student from Boston University wrote recently.

Roommate: Her boyfriend was a problem

About 10 inches of snow blanket the field at Innis Grove Park in Ames. Tong and her roommate came here on their last Sunday together with about 20 others from their church. They ate stir fry vegetables, mixed fruit, grilled meat and hot dogs. Tong brought frosted cookies.

While others played kickball, Tong quietly nibbled on her treats.

"I never imagined that would be my last memory," says Jean, who asked that only her American name be used for her own security.

Tong Shao liked to cook for friends. Her roommate says her boyfriend was a point of friction among them.

The snow serves as a reminder of another memory. At the end of last winter, Jean and Tong tried to make a snowman after a storm swept through. The snow was soft, not wet enough to make a snowball. The two went to YouTube and learned how to make a proper snowman. They pledged to try it during the next snowfall.

"We were going to do it again this year," Jean says, before adding softly, "But it never worked out."

The killing has taught her to appreciate special moments, to live for today because you never know when will be your last. "One day you're here," she says, "the next ..."

Tong was known as "Little Sister." At 20, she was the oldest of three roommates, but her nickname stuck because of her diminutive 5-foot-2 stature.

Jean laughs when looking at a family photograph of her roommate from 2007. It shows Tong dressed in a T-shirt and with short hair. Tong embraced America and shed T-shirts for clothes with frills. Her hair was long, almost down to her waist, with a dyed brown streak and curls. "That is the her I remember."

In the classroom, she was one of those supersmart students who drove the others crazy. You know, the smart kid who always complains of being behind and then gets straight A's.

"I'd be, like, seriously! That's not cool."

Jean and Tong met their freshmen year. Their friendship blossomed over the next two years. When Jean introduced her roommate to friends at church, Tong interrupted: "We're friends first; roommates, second."

"That has always stuck with me," Jean says.

Known as "Little Sister," Tong wore her hair long in the U.S. "She is such a girlie person," her roommate recalls.

There was one point of friction in their friendship: Tong's boyfriend, Xiangnan Li.

Jean didn't like him. He moved into their apartment in summer 2013. She says he didn't ask permission, and never offered to help clean.

"We wanted to kick him out, but he refused to leave," she says. "Tong knew we didn't like him, so she didn't talk about him around us."

"We just didn't want a guy in our apartment. It's not normal."

She never had a confrontation with Li, but she let her roommate know she believed he was a jerk.

Tong and Li had met while taking English prep classes in Beijing in summer 2011. He traveled to Tong's hometown of Dalian, a coastal city of 7 million about 300 miles east of Beijing. He even met her mother briefly, although the two didn't speak. Mom and Dad didn't think too much about the courtship. Tong rarely talked with them about Li, saying only that he was "chasing her."

"She didn't tell us much about their relationship," her father says.

For Tong's parents, images of small-town America brought comfort. Iowa was home to John Wayne and the artist Grant Wood, whose iconic portrait of a farmer with a pitchfork, "American Gothic," was inspired while visiting there for an art exhibition. Mom and Dad preferred sending their only child to a small college town in middle America, rather than a university in a cosmopolitan city.

Li was from Wenzhou, a city of about 9 million on the east coast of China, about 300 miles south of Shanghai. He was majoring in business in Iowa City, about 140 miles from where Tong studied, in Ames.

Li had transferred to the University of Iowa from Rochester Institute of Technology to be closer to Tong. He lived in Tong's apartment until his classes began in Iowa City in fall 2013.

He is mostly a mystery man to the tight-knit Chinese community at the university. He lived alone and off campus. He had few friends, but that's not entirely unusual because he had only been at the university for a year. One friend said he was not very outgoing, although he would invite a few people to his apartment.

It seemed he mingled mostly with his girlfriend across the state, and it wasn't unusual for the couple to disappear on weekends. He'd cruise over in his 2009 blue 325i BMW. The couple knew his presence wasn't welcome at the apartment, so they'd find a hotel.

Jean wishes she'd realized something was wrong when her roommate didn't return home after that weekend. She now clings to a prayer book Tong gave her.

She opens it to the last day she saw her friend alive.

"Keep me focused, God, on the path before me, even when it seems stacked with insurmountable obstacles," it says. "Remind me that I do not walk alone, but with all those whose aim is to walk with you."

Boyfriend's last known message: 'Fine for now'

Mountains of shoveled snow surround the parking lot of the Budget Inn and Suites in Nevada, Iowa, just outside Ames. Tan and green vinyl siding decorate the side of the hotel -- not a fancy place, more of a spot for a quick night's rest or a hookup.

Tong and Li checked into Room 218 on September 5, 2014, the weekend after Labor Day. The owner of the hotel told police they arrived in a gold-colored car, one similar to the description of Tong's beige 1997 Toyota Camry.

The couple was familiar with the hotel. They stayed there three other times: in September 2013, October 2013 and May 2014.

David Gonzalez, the lead investigator on the case with the Iowa City Police Department, said he wishes Li would come forward to answer questions. "What he was doing, the relationship, where he was going, why he decided to leave -- those things I'd like to ask," he says.

Li's cellphone no longer works. When CNN called the home of his parents, a woman answered and immediately hung up when told a reporter was calling about the whereabouts of Li.

Tong visited Disney World in Orlando while on a break from school last year.

Police records unsealed in January show Li may have been angry and jealous that weekend. Just two days earlier, he called Tong; she didn't realize it, but she answered and he stayed on the line for 30 minutes, overhearing her conversation with a friend, the records show.

"Li heard (Tong) complaining about Li and saying things about him that were not nice," according to police records.

The owner of the hotel told police that Li left the hotel in the overnight hours of September 6 and into the morning of September 7. That was unusual, according to the owner, because he typically had to ask Li and Tong to leave the room after the checkout time of 11 a.m.

Li had purchased a plane ticket to China and by Monday morning, September 8, according to police records, he boarded a flight in Cedar Rapids, with a layover in Chicago. He landed in China two days later, on September 10.

Before Li left Iowa, a text from his phone was sent to one of Tong's roommates. "This message was purportedly from (Tong) and read that Li had an emergency in China and was flying back there, that she was going to take a bus to Minnesota to visit friends, and she would return in about a week," the police documents says.

Karen Yang, a friend of Li's who was interviewed by police, sent him a message on September 9, asking how things with Tong were going. "Fine for now," he responded, according to police records. CNN reached out to Yang, but did not hear back.

Tong's roommate, Jean, filed a missing person report on September 18. She and her roommates hadn't been too concerned in the days prior, because they thought Tong traveled to Minnesota as the text indicated. Her roommates combed through Tong's friends on social media in Minnesota and reached out to each one: No one had heard from her.

"That's when we started to get really worried," Jean says.

On September 26, a stench wafted from a car parked in a cluster of apartments on the outskirts of Iowa City. The apartment buildings, more than 40 in all, sit in rows, like military barracks, and are home to some of the town's poorest residents.

Tong's Toyota Camry was parked under a tree near Building 44. Inside the trunk, police found Tong and a 15-pound barbell next to her. Workers with the medical examiner's office wore hazmat suits at the scene.

Tong died of asphyxiation and blunt force trauma. A towel from the hotel was wrapped around her head, according to police records.

Among heaps of stuff in the backseat, police found copies of Li's flight information.

When police finally examined Room 218 at the Budget Inn, the walls "had splatters and drips of various dried liquids throughout the room and behind the headboards of the beds," police records say.

"It is possible her death may have occurred in the hotel room."

By that time, Li was long gone.

In his apartment were signs of a man who left in a hurry. Milk spoiled on the counter; a packed suitcase was left by the front door.

Father receives news of arrest warrant

Tong Shao vacations with her mother and father in 2007. Her father says the trip remains his favorite memory.

CNN first spoke with Tong Shao's family in early February. Her father says he talked with his daughter for the last time on September 3. It was an ordinary conversation: She said she lost her glasses and would try to get a new pair.

He exchanged text messages with her on the night of September 7, possibly the day she was killed. He asked about the glasses; she said she'd borrowed a pair from a friend.

"She told me she was too busy to video talk with me," he says. "I didn't think much of it. But in retrospect I realize those messages weren't sent by my daughter; they were from Li. Once she said she was busy, I didn't insist on video chatting again, as I didn't want to bother her.

"I never thought I wouldn't be able to talk to her anymore."

He spoke with Li's parents shortly after his daughter was reported missing. He said the parents told him they had not heard from their son. "They told us that they didn't know where he was, either," he says. "We have not contacted his parents since Tong was confirmed dead."

The news was so devastating that Tong's father watched his blood pressure skyrocket; he couldn't travel to the United States to escort her ashes home. He cries when he looks at her smiling photographs around the home. He cherishes a trip in 2007 to Yunnan Province when the family glided down a river on a boat and took in other beautiful sights.

His psychologist has suggested the father practice Buddhism, hoping that the faith could help ease his suffering. "I can't stop thinking about this whole thing," he says. "I think about it all the time, and dream of it at night."

He and his wife had planned to travel to the United States for Tong's graduation next May.

About a week after he first spoke with CNN, the father received a call from police in Iowa City. He was informed that an arrest warrant has been issued for Li; he is wanted on a charge of first-degree murder.

In a sign of how sensitive the case is, the prosecutor and police in Iowa City would not confirm or deny the father's account. A provision of Iowa law allows for a warrant to remain under seal under special circumstances until an arrest is made and prevents any official with knowledge of the case from speaking publicly about it.

Police would only say they had spoken with the father.

For a father and mother thousands of miles away, the arrest warrant signifies hope.

"We don't know if our health condition will allow us to wait until the day when justice prevails," the father says. "We hope that the case will be resolved as soon as possible."

The next phone call, he hopes, is news of an arrest.