Walter Espinoza, a government spokesman said in a press conference that they still can not provide any details or confirm that the body belongs to 36-year-old Stefaniak.
Her sister-in-law, April Burton, said she last heard from Stefaniak around 8 p.m. last Tuesday.
Costa Rican authorities believe a partially decomposed body found Monday is Carla Stefaniak, a 36-year-old U.S. citizen who was reported missing last week, said Walter Espinoza, of Costa Rica's Judicial Investigation Department.
Espinoza added that authorities found stains in the Airbnb apartment where Stefaniak was staying that were "compatible with blood", though they are conducting further testing.
Stefaniak's brother flew to Costa Rica last Friday to search for her. "Alarmed, they discovered Stefaniak had never boarded her flight home, which was scheduled for departure around 1 p.m. on November 28". It was discovered partially buried in plastic bags in a mountainous area.
The two texted each other via WhatsApp throughout the day that Tuesday, Burton said, until Stefaniak suddenly stopped responding to her messages that evening.
When she got there, she FaceTimed a friend.
Burton, however, left November 27, a day before Stefaniak was scheduled to leave.
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Authorities said a 32-year-old Nicaraguan man, identified as Bismark Espinosa Martinez, has been arrested in connection with the slaying of Carla Stefaniak, 36, of Hallandale Beach.
"That's when we knew that something's not right", Burton said.
Authorities are waiting for autopsy results, which could be released Tuesday. Airbnb told CBS News it has no comment, and Uber said it will assist local authorities.
"This is a complex, hard case, and we're doing all that we can within our ability to solve the case", Espinoza said at a news conference.
"The guards (at the apartment complex that housed the Airbnb) told us that she left at 5 in the morning with all of her bags and got into an Uber she wouldn't do that her flight was at 1:30 p.m. she's never been early for anything in her life she wouldn't leave to go to the airport at 5:30 a.m.", Burton told ABC Action News.
She wrote that it was raining there and the lights were flickering on and off.
Burton had previously warned that Stefaniak may have been kidnapped as an Uber driver who was scheduled to drive her to the airport said she had taken another vehicle there.
"Six o'clock, still haven't heard from her", Burton said. We will always be with her and we know she will always be with us. When she never exited her scheduled flight Wednesday, her family began worrying. The last time she posted to social media accounts was the previous night.