A US judge has quashed the conviction of a man who has served more than two decades in prison for his ex-girlfriend's murder -- a case that received worldwide attention thanks to the hit podcast "Serial."

Baltimore City Circuit Court Judge Melissa Phinn vacated the conviction of Adnan Syed, 42, who has been serving a life sentence since 2000 for the 1999 murder of Hae Min Lee. 

Phinn ordered Syed released immediately on Monday "in the interests of justice and fairness."

Cheers erupted in the packed courtroom when the judge ordered officers to "remove the shackles" from Syed, who was sporting a thick beard and wearing a white shirt, dark tie and a white skullcap.

Lee's body was found buried in February 1999 in a shallow grave in the woods of Baltimore, Maryland. The 18-year-old had been strangled.

Syed has steadfastly maintained his innocence but his multiple appeals had been denied, including by the US Supreme Court which declined in 2019 to hear his case.

In a surprise move last week, the Baltimore City state's attorney, Marilyn Mosby, announced that she had asked the court to vacate Syed's conviction while a further investigation is carried out.

Assistant state's attorney Becky Feldman told the judge on Monday the decision was prompted by the discovery of new information regarding two alternative suspects and the unreliability of cell phone data used to convict Syed.

"The state has lost confidence in the integrity of his conviction," Feldman said. "We need to make sure we hold the correct person accountable.

"We will be continuing our investigation," she said, while promising to "do everything we can to bring justice to the Lee family."

Erica Suter, Adnan Syed's attorney, speaks outside the courthouse in Baltimore. Photo: AFP

Syed's attorney, Erica Suter, also addressed the court, saying "my client is innocent."

Suter was asked by reporters how Syed, who did not make any public statement, reacted to the judge's decision.

"He said he could not believe it's real," she said. 

'Blindsided' 

Baltimore City prosecutors now have 30 days to either bring new charges against Syed or dismiss the case.

"We're not yet declaring Adnan Syed is innocent," Mosby, the state's attorney, told reporters after the hearing.

She said the state was awaiting the results of new DNA tests on Lee's clothing before deciding whether to drop all charges or organize a new trial.   

Before the hearing began, Lee's brother, Young Lee, addressed the court by Zoom.

An emotional Lee said he was "kind of blindsided" by the prosecutor's decision to vacate Syed's conviction.

"Out of nowhere I hear that there's a motion to vacate judgment," he said. "It's tough going through this again and again and again."

Lee said he "trusts the court system" and asked the judge to "make the right decision."

Syed's case earned worldwide attention when it was taken up in 2014 by "Serial," a weekly podcast that saw a journalist revisit his conviction and cast doubt on his guilt.

His case has also been the subject of a four-part documentary on the HBO channel called "The Case Against Adnan Syed."

The "Serial" podcast -- a mix of investigative journalism, first-person narrative and dramatic storytelling -- focused its first season on Syed's story in 12 nail-biting episodes.

Both Syed and Lee were high school honor students and children from immigrant families -- he Pakistani, she South Korean -- who had concealed their relationship from their conservative parents.

Prosecutors said during the trial that Syed was a scorned lover who felt humiliated after Lee broke up with him.