BANGKOK — For two years, Yingluck Shinawatra, the former prime minister of Thailand, battled criminal charges of negligence and claims that she had cost the country billions of dollars. She appeared in court regularly and assured her supporters that she would fight until the end.

“You need to wait and see,” she told The Bangkok Post last month. “I’ll be there in court to the last day. We will meet there.”

But on Friday, when the time arrived for the Supreme Court to render its verdict, Ms. Yingluck was nowhere to be found. She sent word to the court that she was ill. But senior members of her party say they believe she slipped out of the country rather than face the prospect of spending up to 10 years in prison.

Her whereabouts remained a mystery on Sunday. Officials of her party, Pheu Thai, said they had not heard from her and had no idea where she was.