Carla Stefaniak was last seen in a suburb outside the capital of San Jose, authorities said.The last time anyone heard from Stefaniak was Wednesday night, according to her sister-in-law, April Burton. The two were there to celebrate Stefaniak's birthday, and Burton had been traveling with her through Costa Rica for six days.Burton left the country on Wednesday, and Stefaniak's flight back to Florida was scheduled for the day after. The two were texting via WhatsApp throughout the day on Wednesday, Burton said, until Stefaniak suddenly stopped responding to her messages that evening. And on Thursday, Stefaniak did not board her flight home."That's when we knew that something's not right," Burton said. "You never want to think the worst, but that's where your mind goes to automatically. We're going on six days not hearing from her. That's kind of where we are right now." Burton continued: "For almost six days to go by and no contact, she would have to have been abducted. She is very active on social media, and for her, on the day of her birthday, to never log in to any of those accounts, is just crazy." A spokesperson from the State Department told CNN they are aware of the reports of a missing US citizen in Costa Rica."The US Department of State and our embassies and consulates abroad have no greater responsibility than the protection of US citizens overseas," the spokesperson said. "When a US citizen is missing, we work closely with local authorities as they carry out their search efforts. We stand ready to provide all possible assistance to US citizens in need and to their families. Due to privacy considerations, we have no further comment."Speaking to HLN on Monday, Burton urged anyone who might have information about her friend's whereabouts to come forward."She's a great person. So friendly, so bubbly, loves to travel, loves fashion, loves social media," she said.
CNN's Eric Levenson contributed to this report.