Lee, Syed's ex-girlfriend and former Woodlawn High School classmate, went missing in January 1999.
Chapman said she was in the library next to their high school when she saw Syed at the time Lee was said to have been murdered. A month later, her body was found in a Baltimore-area park.
Syed's current attorney, Justin Brown, tweeted out a jubilant, "We won the appeal", on Thursday after the court upheld the decision to grant Syed a new trial.
Chinese nationals living in the United States have also begun to disappear under suspicious circumstances, although unlike previous targets, "they were not high-profile folks", said one former US intelligence official.
Baltimore Circuit Judge Martin Welch had ruled that Syed's original lawyer, Cristina Gutierrez, should have cross-examined a witness about the reliability of cellphone tower evidence. Maryland Attorney General Brian Frosh had appealed the lower court's 2016 decision.
Syed, who was the focus of the popular 2014 podcast 'Serial, ' has spent the last 18 years behind bars.
"We are now reviewing today's decision to determine next steps", Raquel Coombs, a spokeswoman for the Maryland Attorney General Brian Frosh, said in an email.
Syed maintained his innocence, and in 2014 his case attracted worldwide attention when it was featured on the "Serial" podcast, which was downloaded millions of times and prompted legions of listeners to scrutinize the case online.
"How can you possibly evaluate an alibi witness without speaking to her?"
The show attracted millions of listeners and shattered records for the number of times a podcast has been streamed or downloaded. The podcast "was enormously helpful" in garnering support for Syed, his lawyer Brown said at a press conference Thursday. "We're ready to try the case", Brown said. She said at the time that she could no longer practice because of ill health.
Brown said he is considering whether to seek Syed's release on bail.
This story is developing.