The Supreme Court declined on Monday to hear the appeal of Brendan Dassey, whose murder conviction was documented in the 2015 Netflix documentary series “Making a Murderer.”

His lawyers, and a legion of supporters he gained after the popular series questioned his 2007 conviction, argued that police investigators improperly coerced his videotaped confession. Mr. Dassey, 28, was convicted of participating with his uncle Steven Avery in the 2005 murder of Teresa Halbach, a 25-year-old photographer in Manitowoc, Wis.

The 10-part Netflix series by the filmmakers Laura Ricciardi and Moira Demos suggested that police investigators unfairly questioned Mr. Dassey when he was 16 without a lawyer or parent present. He appeared scared and unaware of the gravity of his situation on camera, and his lawyers say he had a low IQ in the seventh percentile of children his age, making him susceptible to suggestion.

The Supreme Court did not offer a reasoning for its decision.

In a statement, Laura Nirider, a lawyer for Mr. Dassey, said, “We will continue to fight to free Brendan Dassey.”