As Chinese President Xi Jinping left for the US on an official visit, Beijing yesterday said the country was investigating an American woman for spying, an allegation her husband in Texas said was groundless.

Sandy Phan-Gillis of Houston, Texas, has been held by Chinese authorities for about six months under suspicion of spying and stealing state secrets, according to a statement from her family released this week.

“I have absolutely no clue whatsoever why she was taken into custody,” her husband, Jeff Gillis, said from their home n Houston, adding “Sandy is not a spy or a thief”.

Phan-Gillis ran a consulting business that helped pair US and Chinese firms and had been to China numerous times without incident, Gillis said, adding he had asked the State Department for months to press for her release, but to no avail.

“My intent here was not to embarrass China or President Xi Jinping. It was really to encourage them to release Sandy in the lead-up to the state visit,” he said.

“I concluded from my research that if I remain quiet and do nothing, that my wife is just going to end up being quietly crushed by a Chinese criminal justice system that has absolutely no justice in it, or no real rule of law,” he added.

China’s state secrets law is notoriously broad, covering everything from industry data to the exact birth dates of state leaders. Information can also be labelled a state secret retroactively.

Chinese Foreign Ministry spokesman Hong Lei said Phan-Gillis was suspected of “endangering China’s national security” and is being investigated by “relevant Chinese authorities”.

The US State Department said it was aware of her detention and is monitoring her case.

China has permitted her at least six consular visits and she is in good health and cooperating with the investigation, Hong told a news briefing.

In a statement, State Department spokeswoman Katy Bondy said Phan-Gillis was arrested on March 20 and confirmed the US consulate visits. It referred questions about her arrest and any charges to local authorities. Chinese authorities did not say if she has been officially charged in the case.

“We continue to monitor her case closely,” Bondy said.

China’s Ministry of State Security could not be reached for comment.

Phan-Gillis, who has Chinese ancestry and is a naturalised US citizen, was also the head of the Houston Shenzhen Sister City Association. She visited China on a trade delegation from Houston and was detained while attempting to cross from the southern city of Zhuhai to Macau on March 19, according to the family statement.

Gillis said he spoke to his wife in the early days after being taken into custody, with the last conversation on March 23. At that time, she gave no indication she had been apprehended by Chinese authorities, he said.

Phan-Gillis is being held in the southwestern city of Nanning, the family said. She had been under house arrest until she was transferred to a detention centre on Saturday, it added.

It was unclear whether any formal charges have been brought. A lawyer working on her case could not be immediately reached.

“Sandy is in very poor health,” the family statement said, adding that she suffers from high blood pressure, high cholesterol and high blood sugar. She has been hospitalised repeatedly while detained, it said.