A 55-year-old Texas businesswoman of Chinese heritage, who was detained while attempting to enter Macau in March during a visit to China, is being investigated for "endangering China's national security," China's Foreign Ministry announced on Tuesday.

According to Foreign Ministry's spokesperson Hong Lei, the suspect Sandy Phan-Gillis is "well and healthy." He also noted that her rights are being respected in custody and that she is cooperating with investigators.

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"We hope that the outside world will respect China's handling of this case according to law," Hong said.

Meanwhile, Sandy's husband Jeff Gillis has denied that she is a spy. According to a website created to spread awareness about her case, Sandy was in China to promote business between the country and the American city of Houston - where she lives. Sandy is said to be a member of Houston's Mayor's International Trade and Development Council as well as the President of the Houston Shenzhen Sister Cities Association.

It is unclear what the exact charges against her is and why she has been detained for so long. According to the Houston Chronicle, soon after Sandy was detained, she told the party she was travelling with that they should go ahead because she is attending to some personal matter. She allegedly also told her husband initially that she would be staying in China for longer than expected.

Gillis emphasizes that his wife is not a "spy or a thief."

The website calling for Sandy's release says she has "poor health" and has to take several medications daily. She allegedly suffers from high blood sugar, high cholesterol and high blood pressure. It further claims that she has been rushed to the Emergency Room at least twice since she was detained.

Until recently, Sandy had been placed under house arrest. There are speculations that her transfer to a detention center could mean that the Chinese government is finally ready to level charges against her.

Gillis has urged U.S. authorities to take advantage of President Xi's visit to secure the release of his wife.

TagsSandy Phan-Gillis, Jeff Gillis, Houston Shenzhen Sister Cities Association, #SaveSandy