Their stories have stumped investigators, agonized families and captivated the American public. Following are the names of some of the highest-profile crime victims whose cases remain unsolved to this day.

Jennifer Kesse

Known by friends and family simply as "Jenn," the 24-year-old Orlando woman was last seen alive on Jan. 23, 2006. Kesse, who lived alone, was reported missing by her parents the next day when she failed to show up for her job as a manager at Central Florida Investments.

Kesse’s car – a black Chevy Malibu – was found by authorities on Jan. 26 approximately one mile from her condominium. The Orlando Police Department had hoped for a break in the case when it released security footage of a person seen parking Kesse’s vehicle near a pool at an apartment complex and walking away. But the suspect – who appears to be between 5-foot-3 and 5-foot-5 – has never been identified.

In a recent interview with FoxNews.com, Drew Kesse, Jennifer's father, said he believes more than one person is responsible for his daughter’s disappearance. His theory is that Jenn was abducted while leaving her residence early on Jan. 24, and that she never made it to her car.

No forensic evidence was obtained from her vehicle – only one “latent print,” which her father called “too minuscule” to be useful. The last known police search for Kesse -- who is 5-foot-8 with blond hair -- was conducted in February, when investigators scoured an overgrown field, yielding no clues in the case.

Kesse suggested his daughter may have been taken by human traffickers and may no longer be in the country. “I think that someone wanted a prize,” he told FoxNews.com. “Over 1,300 leads now and every single one of them is a dead end.”

“She had no enemies,” Kesse said of his daughter, a 2003 graduate of the University of Central Florida in Orlando and an Alpha Delta Pi sorority member he described as driven and outgoing.

“We don’t own a picture without her smiling,” he said.

Robert Krentz

Robert N. Krentz Jr., a prominent rancher whose family has been ranching in southern Arizona since 1907, was shot to death on March 27, 2010. Investigators say the 58-year-old Krentz was found dead inside his all-terrain vehicle on his 35,000-acre Cochise County ranch with multiple gunshot wounds. Krentz’s dog, Blue, was found critically injured in the back of the ATV.

Authorities say Krentz's body was found several hours after he radioed his brother, Phil, saying he was setting out to help someone on the ranch who he believed was an illegal immigrant.

"He was a good and kind and gentle person," Krentz’s wife, Sue, said in an interview. "Somebody chose to invade our lives and create a crime that was unjustified. It’s torn a big hole in our family.”

Krentz spoke to his brother sometime between 10 a.m. and 10:30 a.m. on March 27, according to police. Phil Krentz said he was able to make out the words “illegal alien” and “hurt” during that exchange. The brothers had planned to meet at  noon, but Robert Krentz never showed up. At approximately 6:20 p.m., family members reported him missing.

Investigators were able to trace Krentz’s vehicle back about 1,000 feet to where they claim the incident began. Detectives say they believe Krentz and his dog were shot inside their vehicle after coming upon his assailant. They say the rancher then drove off at a high speed before falling unconscious.

Sherriff’s deputies and the U.S. Border Patrol identified a single pair of foot tracks that led them approximately 20 miles south of the U.S. border with Mexico.

The find prompted authorities as well as Krentz’s family to say they believed he encountered a drug smuggler returning to Mexico. To date, no suspect has been named.

In a recent interview with FoxNews.com, investigative sources declined to comment on any possible advances in the case, saying only that "interesting leads have been developing" and that authorities remain committed to catching the killer.

Sue Krentz said it was not unusual for her and her husband to help the illegal immigrants they encountered on their vast stretch of land bordering Mexico.

“We’ve had lots of illegals on our property before so we were never afraid of them and never assumed that they would hurt us,” she said. “I’m just hoping someone would have a conscience and just admit it or turn themselves in.”

“It’s the first time in 33 years that he won’t be here for Christmas,” Krentz said.

Natalee Holloway

The 18-year-old high school graduate from Mountain Brook, Ala., disappeared in May 2005 while on a school trip to Aruba. The honor student’s body has never been found.

Joran van der Sloot, a 22-year-old Dutch playboy and the lead suspect in Holloway’s disappearance, is currently being held in Peru -- charged with murdering a 22-year-old Peruvian woman in May 2010.

According to friends’ accounts, Hollway was last seen leaving Carlos'n Charlie's -- a popular hangout in Oranjestad -- at around 1:30 a.m. on May 30 in a car with Van Der Sloot and brothers Deepak and Satish Kalpoe. Holloway failed to show up for her return flight to Alabama later that day.

An unprecedented search of the island – including help from the FBI, the Dutch Army and F-16 aircraft -- yielded no sign of the 5-foot-4 blonde, who had plans to attend the University of Alabama on full scholarship.

Van Der Sloot’s version of events surrounding Holloway’s disappearance has changed several times. He first told authorities that he and the Kalpoe brothers drove Holloway to a lighthouse before dropping her off at the Holiday Inn, where Van Der Sloot said she stumbled as she left the car and was approached by a dark-skinned man. He later modified his story, saying he left Holloway alone on a beach. In 2008, in an interview with Fox News, Van Der Sloot confessed to selling Holloway into sex slavery (an admission he later retracted). The Dutch national also claimed he was under the influence of marijuana when a hidden camera captured him telling a friend that Holloway died after she had a seizure.

Van Der Sloot was arrested twice on suspicion of involvement in the case and then released by Aruban authorities. On June 3, 2010, he was indicted by a federal grand jury in the U.S. for wire fraud and extortion related to Holloway's disappearance. He allegedly demanded $250,000 from Holloway's mother, Beth Twitty, in exchange for information on the whereabouts of her daughter's remains.

Van Der Sloot was arrested that same day by Peruvian authorities for allegedly beating Stephany Flores Ramirez to death in a hotel room in Lima hotel on May 30 -- the fifth year anniversary of Holloway's disappearance.

Twitty reportedly confronted Van Der Sloot after sneaking into Castro Castro Prison last month. "I want to know what happened and I want to move on, Joran," Twitty is seen telling him on tape. "I want to move on in my life and I can't close the book."

Investigators hoped that a human jawbone found on an Aruban beach last month would provide a break in the case, but Aruban authorities determined that the remains were not Holloway's.

David Hartley

David Hartley, 30, of Milliken, Colo., was shot and killed Sept. 30, 2010, while jet-skiing with his wife on Lake Falcon, a 25-mile-long lake along the U.S.-Mexico border that is notorious for drug smuggling.

Tiffany Hartley says her husband was shot in the back of the head while the couple was dodging bullets from three speedboats containing Mexican drug cartel members. She said they encountered the killers when they set out set on their jet skis to photograph a half-submerged church on the Mexican side of the lake. The 29-year-old said she was forced to leave her husband lying face down in the water because she was unable to pull him onto her watercraft.

Authorities found blood on Tiffany Hartley’s life vest, but her husband's body was never recovered. A police source speaking on condition of anonymity said confidential informants indicated that the body and jet ski were destroyed and will never be found.

A Mexican state police commander investigating the shooting was found decapitated in October – his severed head delivered in a suitcase to the Mexican military.

After pressure from U.S. officials, Mexican authorities in October identified two men as suspects in Hartley's murder -- Juan Pedro Saldivar Farias and his brother, Jose Manuel, both from New Guerrero, Mexico, and members of the Zeta drug cartel .

In an interview with FoxNews.com, Zapata County Sheriff Sigifredo Gonzalez said he recently received information identifying two additional suspects in the murder. Gonzalez said the men, Zeta drug cartel members whom he did not identify, indicated that they disposed of Hartley’s body and his jet ski.

Gonzalez said he also has information that the shooters were ordered to kill Tiffany Hartley, and he said he believes at least six drug cartel members – and possibly eight – were on the lake when the shooting occurred.

"Information has been revealed that [they] authorized her killing as well, but she was able to escape,” Gonzalez said. “They were targeted because they were in the wrong place at the wrong time.”

Gonzalez expressed frustration with Mexican law enforcement, saying sometimes “they just won’t call back,” but said he’s optimistic the killers will be apprehended.

“Eventually something’s going to happen,” he said, “Information is constantly flowing.”

Kyron Hormon

The largest missing person search in Oregon's history was launched for Kyron Horman, an 8-year-old Portland-area boy who disappeared June 4, 2010.

The child’s stepmother, Terri Moulton Horman, told police she last saw the boy walking down the hallway of Skyline Elementary School on his way to class after having attended a science fair. School officials said the second-grader never made it to class.

No person has been charged or made a person of interest in Kyron’s disappearance, Lt. Mary Lindstrand of the Multnomah County Sheriff told FoxNews.com.

Terri Horman, 40, said she accompanied her stepson to school early June 4 in her white pickup truck. She said the two previewed the school's science fair exhibits -- along with other parents and children -- before Kyron's first class was to begin. Terri then reportedly posted a picture on Facebook of the smiling boy standing in front of his tree frog exhibit.

She called 911 at approximately 3:45 p.m. after Kyron, who was last seen wearing a black T-shirt with the letters "CSI," failed to return home on his school bus.

Hundreds of Oregon law enforcement officials -- with help from the FBI and the Portland Police Bureau's air unit -- searched for the boy.

Questions arose over the truthfulness of Terri Hormon's account after she was asked by authorities to take a polygraph test twice. Her cell phone records also didn't match where she said she was on the day Kyron disappeared. The child's biological mother, Desiree Horman, later called on her to "fully corporate with the investigators" to bring Kyron home.

Kaine Horman, the child's father, filed for a divorce against Terri, stating in court documents that it was common for his wife to be impaired by alcohol "several nights a week." Police also told Kaine Horman that about six months before Kyron disappeared, Terri Horman allegedly hired a landscaper to kill her husband.

But Lindstrand told FoxNews.com that Terri Horman was "never in custody" and investigators are still pursuing all leads in the case. Searchers combed an island about 15 miles outside of Portland in October looking for any signs of Kyron.

"We’re still very hopeful that there will be closure on this case," Lindstrand said.

SLIDESHOW: Click here to see profiles of more unsolved mysteries.