The Supreme Court declined on Monday to hear the case of Adnan Syed, whose murder conviction nearly 20 years ago formed the basis of the hit podcast “Serial.”

Mr. Syed had petitioned for a new trial, arguing that his original lawyer failed to pursue an alibi witness. But the justices left in place a ruling by Maryland’s highest court that denied his petition. They did not comment on the decision.

Mr. Syed is serving a life sentence for the 1999 killing of Hae Min Lee, a high school classmate he had dated whose body was found in a park in Baltimore County. Both were teenagers when the crime occurred.

Mr. Syed has maintained that he is innocent. He was convicted of first-degree murder, robbery, kidnapping and false imprisonment in the death of Ms. Lee.