Updated at 10:45 a.m. ET

The U.S. Supreme Court announced Monday that it will not consider an appeal filed by convicted killer Adnan Syed, the main subject of the Serial podcast, leaving in place a state appeals court decision keeping him in prison for life.

The high court did not provide an explanation for why justices declined to hear Syed's appeal.

Syed is serving a life sentence after a jury convicted him in 2000 of strangling to death his former girlfriend, 17-year-old Hae Min Lee, whose body was found in Baltimore.

Syed's case attracted an international following when it was featured in the 12-part podcast in 2014. His case seemed to have a breakthrough in 2016 when a Maryland court ordered a new trial, citing an alibi witness that was never explored by Syed's defense attorney. The Maryland Court of Special Appeals agreed with the lower court.

But state prosecutors continued to fight it. And in March, the state's highest court, the Court of Appeals, determined in a 4-to-3 decision that Syed does not deserve a new trial, reversing his legal victory and keeping Syed locked up. The appeals court did believe that Syed's trial counsel was deficient but did not find any prejudice. That ruling essentially was affirmed by the U.S. Supreme Court.

Syed's lawyer, Justin Brown, said he was "extremely disappointed" by the Supreme Court's decision.

"Two courts have said he deserves a new trial, and then Maryland's highest court reversed that. We think it's appalling," Brown said. "We think this is a grave injustice. The way this has played out is sickening."

However, Brown said Syed's legal fight isn't over — that his client was a juvenile sentenced to life in prison without a "meaningful opportunity for parole."

"That can be litigated in state court," Brown said. "We won't give up."

In the petition to the U.S. Supreme Court, Brown wrote that the prosecution's witness had an inconsistent story and an alibi witness who could have exonerated Syed never testified at the trial.

"Syed is entitled to a new trial," Brown wrote. Syed should have the opportunity to present his alibi to a jury, and the jury — not the Maryland Court of Appeals — should decide its worth."

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