BALTIMORE -- After spending 16 years in prison for the killing of his former high school girlfriend, a man at the centre of the popular podcast "Serial" has a chance at freedom.

Retired Baltimore Circuit Judge Martin Welch ruled Thursday that Adnan Syed deserved a new trial because his lawyer didn't challenge expert testimony in the case that became the focus of the podcast, which captivated millions of listeners worldwide.

Syed, now 35, was convicted in 2000 of murdering Hae Min Lee a year earlier and burying her in a shallow grave in a park in northwest Baltimore. He was sentenced to life in prison.

Lee's family wrote in a letter that they were "very disappointed" with the judge's ruling and that they continue to grieve.

"We continue to believe justice was done when Mr. Syed was convicted of killing Hae," the family wrote in a letter released by the attorney general's office. "While we continue to put our faith in the courts and hope the decision will be reversed, we are very disappointed by the judge's decision."

At a post-conviction hearing in early February, Syed's attorneys argued he deserved a retrial on grounds that his original attorney, Cristina Gutierrez, didn't contact Asia McClain Chapman, an alibi witness who said she saw Syed at the Woodlawn library about the same time prosecutors say Lee was killed.

Additionally, Syed's current attorneys argued cell tower data linking Syed's phone to the burial site on the day of Lee's murder was misleading because it was presented to jurors without a cover sheet warning that incoming call data was unreliable.

In Welch's order, he disagreed that Gutierrez erred when she failed to contact Chapman, or that prosecutors breached their duty by withholding exculpatory evidence. But Welch did agree that Syed's attorney provided "ineffective assistance for the failure to cross-examine the state's cell tower expert about the reliability of cell tower location evidence" that placed him near the burial site.

The state had argued that because Syed didn't raise the issue of his trial attorney's failure to cross-examine the state's cell tower expert in a prior proceeding, he waived his right to make it an issue now. But the judge ruled that Syed didn't "intelligently or knowingly" waive his right to raise the issue, noting that he never completed his high school degree.

"Requiring a layman who lacks a complete high school education to understand the intricacies of cellular network design and the legal ramifications of trial counsel's failures to challenge the evidence would be inconsistent with the spirit of the Sixth Amendment," the judge wrote.

The judge said the attorney's performance "fell below the standard of reasonable professional judgment" when she failed to confront the state's expert.

At a news conference, Syed's attorney Justin Brown said he "fully expects" the state to appeal the judge's decision. But he said he and the rest of the defence team have "dug our heels in" and remain determined to fight for Syed, including requesting that he be released from jail while awaiting retrial.

"This is obviously an incredible victory," he said. "We know the state is not going to give up, and we're ready."

Brown said earning a new trial in Baltimore is "very difficult."

"This was us getting over the hill," he said. "There's a lot more work to be done, but we're feeling pretty confident."

In a statement, the state attorney general's office said that although Welch ruled in its favour on some issues, "there does appear to be at least one ground that will need to be resolved by the appellate courts."

"The State's responsibility remains to pursue justice, and to defend what it believes is a valid conviction," the statement concludes.

During the extensive hearing, defence attorneys and prosecutors called witnesses and vigorously cross-examined others.

Chapman spent nearly two days on the stand, testifying she and Syed spent about 15 minutes chatting in the library on Jan. 13, 1999, but that despite repeated efforts to reach Syed's defence team at the time with an offer to be an alibi, she was never contacted. Chapman wrote a pair of letters and sent them to Syed in jail days after his arrest, detailing their meeting.

This was Syed's second attempt at a new trial. Welch denied the earlier post-conviction relief bid in 2014 after determining that Gutierrez's decision not to pursue Chapman was the result of reasonable trial strategy, not neglect.

In his order Thursday, Welch reiterated his stance that failing to contact Chapman didn't constitute a failure of duty because her testimony, which pertained to the time of the Lee's murder, had little to do with Syed's defence strategy -- instead pertaining primarily to the timeline of Lee's slaying.

Brown also called witnesses to testify that cell tower data -- an important piece of the state's case against Syed -- should have never been presented to jurors without an instruction sheet warning that any incoming call data is inconclusive.

Brown showed the judge an affidavit from the radio frequency technician who testified at Syed's trial for the prosecution that said his testimony would have been different had he seen the instruction sheet prior to taking the stand. But prosecutors countered that the instructions didn't pertain to any relevant data placing Syed's phone in Leakin Park during the time Lee was buried.

This was the issue that persuaded Welch to grant Syed a new trial.

The judge wrote, "there is a substantial possibility that the result of the proceeding would have been different but for trial counsel's failure to cross-examine the State cell tower witness about the disclaimer."

The podcast, which debuted in the winter of 2014, attracted millions of listeners and shattered records for the number of times a podcast has been streamed and downloaded. The loyal army of listeners often acted as armchair detectives, uncovering new evidence and raising new questions about the case.