The decision issued Thursday by the Court of Special Appeals upheld a lower-court ruling that Syed's counsel in his original murder trial was deficient and ineffective.
Lee, Syed's ex-girlfriend and former Woodlawn High School classmate, went missing in January 1999.
Adnan Syed, a convicted killer who gained worldwide fame from the podcast "Serial", will finally get a new trial after spending roughly 18 years in prison for a murder he says he did not commit. Her body was found a month later in a large Baltimore-area park.
In a 2-1 vote, the Maryland Court of Special Appeals found that the Baltimore man's "Sixth Amendment right to effective assistance of counsel was violated by trial counsel's failure to investigate McClain as a potential alibi witness".
Adnan Syed leaves the Baltimore City Circuit Courthouse in Baltimore, Maryland February 5, 2016. The ruling can still be appealed to the Court of Appeals, according to ABC News.
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Syed, who was the focus of the popular 2014 podcast 'Serial, ' has spent the last 18 years behind bars.
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.
Syed's trial lawyer tweeted the news about the new trial on Thursday afternoon. In a statement to the Sun, the office of Maryland Attorney General Brian Frosh said it was "currently reviewing today's decision to determine next steps". The show attracted millions of listeners and shattered podcast records.
Syed's current lawyer has argued that his original defense attorney, Cristina Gutierrez, had fallen "below the standard of reasonable judgment" during the original trial by not investigating McClain or calling her to testify as an alibi witness.
"In addition to being incredibly entertaining, it was a great piece of reporting by [producer] Sarah Koening", Brown said.