In 2012, U.S. citizen Mark Swidan, a Texas resident with a passion for art and photography, went on a trip to southern China.

More than three years later, he's still there -- in a detention center in Guangdong province -- in an unresolved drugs case that human rights observers say is baffling in its drawn-out nature.

According to an indictment supplied by his family, Mr. Swidan, 41, was detained in November 2012 in the southern city of Dongguan in a large-scale meth manufacturing case after being found with other foreigners at a hotel where drugs were also present. The indictment said Mr. Swidan had introduced the group to someone who knew how to make drugs and had also visited and helped site a factory location where meth was produced.

Mr. Swidan was tried in 2013 and pleaded not guilty. But he has yet to receive a verdict in his case, said his lawyer Kevin Zhang, a partner at the Kingpound Law Firm. Mr. Swidan remains in limbo, still in the same detention center where he was sent more than three years ago.

A photo of art Mark Swidan has produced since his 2012 detention. Illustration: Courtesy of Katherine Swidan

"I'm very upset and don't understand what's happening," said his mother, Katherine Swidan. She says her son is innocent and that he was traveling in the region for business and to secure furnishings for his home in Texas with his future wife.

"I've never seen anything like it, and I go back a long way," said John Kamm of the human rights group Dui Hua Foundation, of Mr. Swidan's case. "It just boggles my mind that an American citizen can be treated like this. His due process rights have been violated very seriously."

According to the World Prison Brief, an online database about prison systems around the world created by the International Centre for Prison Studies, foreigners for around 0.4% of China's prison population.

Chinese law permits verdicts to be delayed if the court applies for and secures permission from higher judicial authorities.

The court has received approval numerous times to delay the verdict in Mr. Swidan's case, which involves nearly a dozen other defendants, including several of Mexican nationality. The most recent delay came last week, when the court was granted a three-month extension to the case, said Mr. Swidan’s mother, citing a notice she received from the local U.S. consulate.

Post-trial delays in verdicts are a regular occurrence in China, with judges sometimes asking prosecutors to furnish more evidence before delivering a judgment, lawyers say. In an indication of how long delays can extend, the Supreme People's Procuratorate in August declared it was planning to crack down on cases of “prolonged detention,” which it defined as suspects being detained for more than five years without a verdict.

A photo of art Mark Swidan has produced since his 2012 detention. Illustration: Courtesy of Katherine Swidan

An employee who answered the phone at the Jiangmen Detention Center, where Mr. Swidan is being detained, said Mr. Swidan wasn't available for comment. A staffer picking up the phone at the Jiangmen City Intermediate People's Court, where the case was heard, said no one was available to comment.

While Mr. Swidan and others were detained in Dongguan, the meth factory was located in Taishan, a region that belongs administratively to Jiangmen, according to the indictment.

Ms. Swidan said that she had been sending her son socks, books and letters. She said she was fearful her son's health would deteriorate inside the detention center, particularly in the cold winter months.

In an emailed response to questions, a U.S. State Department official said that officers regularly visit Mr. Swidan and that the agency had "expressed our concerns about the pace of the proceedings" and was monitoring the case. "We have expressed our desire that Mr. Swidan’s case proceed through the court system in an expeditious and transparent manner, in accordance with Chinese law," the official said.

Mr. Kamm said Mr. Swidan's case should have long ago been dealt with, regardless what the eventual outcome may be. "If he's guilty, put him on trial and convict him and send him to prison," Mr. Kamm said. "If not, let him go."

--Te-Ping Chen. Follow her on Twitter.