Family and friends of an Arizona businesswoman jailed in China for more than three months tell NBC News they are growing increasingly worried for her.

Shiping Jiang Peterson of Fountain Hills, a suburb of Phoenix, was detained on Aug. 29 as soon as her flight landed in Shanghai, her son, Dawei Jiang, told NBC News on Monday. He and Peterson had traveled there to attend her father's funeral.

"Someone came onto the plane before anyone got off," said Jiang, 24. "My mom showed her American passport. They took her phone. My mom asked why are they taking her and they said they would tell her later."

After waiting in China for weeks for news of his mother, he eventually returned to Arizona.

A State Department spokesperson told NBC News it was monitoring the case while providing assistance to her family, and visiting Peterson in the detention facility. They did not provide further details.

"Her life was here," says a friend of Shiping Peterson, a massage therapist from Arizona who is being held by the Chinese government in Shanghai.

China’s Foreign Ministry confirmed to NBC that Peterson had been detained on the grounds of “committing a crime of provocation," but declined to provide any other details on the case.

Peterson's "legal rights have been fully guaranteed," a Foreign Ministry spokesperson said without elaborating.

Under Chinese law, a person who "undermines public order" through "provocative or disturbing behaviors" faces detention or imprisonment.

Peterson, also known as Sue, runs a small business as a licensed massage therapist in the Phoenix suburbs. She was born in China, but has lived in the U.S. for 10 years, her friend Dina Galassini said.

Friends say they are mystified by the allegation and concerned for Peterson's son.

"She is a sweet lady, you can’t tell me she’s a criminal," said friend and neighbor Merita Kraya. "Every time he talks about the situation and what happened, he gets very emotional," she added, referring to Jiang.

"Her life was here," Kraya said.

In a letter to her son sent through her lawyer, Peterson said she was being well treated, Kraya said, although the circumstances around the letter being written were not clear.

The trip was only intended to last four days, she added, saying that Peterson had traveled to China last year without incident.