Syed and Hae were honour students and children from immigrant families who had concealed their relationship from their conservative parents.
Prosecutors laid out a clear case, saying Syed's conservative Muslim upbringing made him feel especially humiliated, but his supporters said authorities had failed to contact a witness who claimed she saw Syed at the time of the murder in a public library.
"I haven't seen a single case in which an attorney failed to contact an alibi witness in which deficiency was not found," his lawyer C. Justin Brown told the Maryland Court of Special Appeals last year.
"This is an alibi witness who is providing their phone number and their willingness to testify, and the reason that this is so important is that there is no more powerful defence than an alibi."
His legal champion and family friend Rabia Chaudry, told the Baltimore Sun in 2016 she believed Syed's Muslim heritage helped the state construct an account of the murder as an "honour killing" after the couple broke up.
"The state framed an argument based on absolutely no evidence that Adnan's honour had been besmirched," she told the Sun.
"They weren't able to find evidence that Adnan was a violent boyfriend or that he had a history of being abusive, so they had to plug in his religion as a substitute."
The case earned new attention when it was taken up by Serial, a weekly podcast in which a US journalist revisited the case and cast doubt on his guilt.
The podcast — a mix of investigative journalism, first-person narrative and dramatic storytelling — focused its first season entirely on Syed's story in 12 nailbiting episodes. They were downloaded more than 175 million times, a world record.
"We conclude that his claim of ineffective assistance of counsel has been established," the Maryland court said in its ruling.
"Accordingly, Syed's murder conviction must be vacated, and because Syed's convictions for kidnapping, robbery, and false imprisonment are predicated on his commission of Hae's murder, these convictions must be vacated as well," it added.
"The instant case will be remanded for a new trial on all charges against Syed."