A Boothwyn teen lured another teen to a secluded park in Delaware County in March with the promise of giving him a gun and instead he killed him, prosecutors said Wednesday.

Mark Laird, 18, has been charged with first- and third-degree murder and firearms offenses. He remained in custody, denied bail. There was no indication he had hired an attorney.

Laird took Reuben Rosado, 15, to Henry Johnson Park in Trainer, saying he would give him a gun he had stashed there, prosecutors said. Once the two teens were alone in the park, prosecutors said, Laird shot Rosado in the back of the head and left him for dead.

Police, responding to reports of gunfire, found Rosado, shot once in the head. He was pronounced dead at the scene.

Investigators learned of Laird’s involvement weeks later from another teen who said Laird had confessed to the murder, saying he was “carrying a burden,” according to the affidavit of probable cause for Laird’s arrest.

The teen told detectives Laird showed up to his home unannounced before it was public knowledge that Rosado had been killed and said he knew what had happened to him, the affidavit said. Laird told the teen he had run into Rosado while out selling drugs and promised to give him a gun he had stashed in the park, the document said.

When the teens were alone in a secluded area, Laird said he sent Rosado ahead to get the gun, and then shot him in the head from behind, according to the affidavit.

Through cell phone records, detectives learned that Laird and Rosado had communicated more than 30 times between January and March, prosecutors said. The records also showed that the two teens were together in the park at the time of the murder.

Laird was taken into custody Tuesday on open bench warrants for unrelated juvenile cases. At the time of his arrest, Laird was walking with another teen who told detectives Laird had confessed to killing Rosado, the affidavit said. The other teen was carrying a ghost gun — a gun assembled from parts that lacks a traceable serial number — that he said Laird had given to him, according to the document.

Laird was carrying a Colt .45 caliber handgun when he was taken into custody, the affidavit said.

Delaware County District Attorney Jack Stollsteimer said Laird’s “cold-blooded execution” of Rosado was “simply incomprehensible.”

“Unable to keep his actions secret, the defendant then admitted to others what he had done,” Stollsteimer said. “The defendant’s actions were senseless, as well as heart breaking.”