By Dakin Andone, CNN

When Carlos Caicedo went to Costa Rica to search for his missing sister, he shook hands with the security guard at the apartment where she was staying and listened to what the man knew, Caicedo told CNN affiliate WPLG.

Little did Caicedo know the man would turn out to be his sister's suspected killer.

"We went to the villa. This guy shook our hands, looks us in the eyes, he tells his fake story," Caicedo said. "And we couldn't feel anything from this guy. Like, I just can't believe it."

Caicedo's sister, Carla Stefaniak, has since been confirmed dead, her body found near the apartment complex where she was staying at the tail end of a vacation to celebrate her 36th birthday.

A Nicaraguan national, Bismarck Espinoza Martinez, 32, has been taken into custody in connection to Stefaniak's death, according to Costa Rican officials. He was a security guard at the apartment complex where Stefaniak's Airbnb was located.

"How cold-hearted can you be?" Caicedo told WPLG. "And such a monster, because you have to be something else, man, to do something like this."

Stefaniak, a US citizen and South Florida resident, went missing late last month while staying at the apartment outside the Costa Rican capital of San Jose.

On Monday, a partially decomposed body was found about 300 yards from the apartment complex, according to Walter Espinoza, a spokesman for Costa Rica's Judicial Investigation Department.

The body showed signs of a violent death, according to her father, who is also named Carlos Caicedo.

"It was a death that was absolutely violent," he said in an interview with CNN en Espaol. "And we ask ourselves why. We have no explanation."

Her family confirmed in a Facebook post early Wednesday morning her father had seen the body and it belonged to Stefaniak.

An autopsy showed multiple stab wounds to the "neck and upper extremities," as well as blunt force trauma to the head, Espinoza, the government spokesman, said.

There were also stains inside the apartment where she stayed "which are compatible with blood and which will be submitted to further investigation and comparison," he said.

Suspect was staying in next apartment, officials say

Espinoza Martinez had been staying in the apartment next to Stefaniak's rental, said Espinoza, the government spokesman.

"The person linked to this homicide (Espinoza Martinez) was close to her and had the time and place to commit the crime," he said.

Detectives who interviewed people who had contact with Espinoza Martinez grew suspicious when they noticed his statements were contradictory, Espinoza said.

The security guard told Stefaniak's relatives he last saw her around 5 a.m. on November 28, getting into an Uber to go to the airport, according to Leandro Fernandez, a friend of the victim.

But that didn't make sense, Fernandez told WSVN, because Stefaniak's flight out wasn't until midday.

The family later learned there were no records in Stefaniak's Uber account from that day, Fernandez said.

Victim was celebrating her birthday

Stefaniak's sister-in-law, April Burton, told CNN they went to Costa Rica to celebrate Stefaniak's 36th birthday. They'd traveled together for six days, staying in beach towns outside San Jose, before Stefaniak dropped Burton off at the airport on November 27 and returned their rental car.

Stefaniak was expected to fly out the next morning, according to CNN affiliate WSVN.

But her relatives grew concerned last week when she stopped contacting them and was not on her flight home from Costa Rica.

A friend of Stefaniak's in New York was the last known person to have contact with her, Caicedo told CNN en Espaol. Stefaniak told her friend she was very thirsty and wanted to leave her Airbnb to get water, but it was raining too hard outside.

Up until that point, "I had constant communication with her," Caicedo said. "She would send videos, texts."

During their travels, Stefaniak told her father, "This is a safe country, and we feel really safe."

Caicedo said he would remember his daughter as a happy and enthusiastic person who was always smiling, he said.

"She had no enemies, none," he said, adding it took him a long time to find a picture of his daughter where she wasn't smiling because "that was the type of woman she was."