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A 17-year-old boy in Indiana has been arrested and charged with murder in connection with the disappearance of two other teenagers, authorities said.

Molley Lanham, 19, and Thomas Grill, 18, were reported missing by their families on Feb. 25, according to the St. John Police Department. Authorities said they believed the teens had been traveling in a black Honda Civic before they vanished.

Prosecutors said on Monday that they arrested Connor Kerner, 17, and charged him as an adult with two counts of murder after two witnesses told the Porter County Sheriff's Office that Kerner had allegedly confessed to the gruesome crimes.

Connor Kerner, 17, was arrested in connection with the murders of two teenagers in Indiana. Porter County Sheriff's Office

One witness told detectives that he was at Kerner's house on Feb. 25 when Kerner said he had done something "really bad" and said he had killed Grill and Lanham, prosecutors said in a charging document released Monday.

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The witness told authorities they do not know Grill or Lanham and initially thought Kerner was joking until they saw a missing person poster for the two teenagers.

According to prosecutors, the witness told detectives that Kerner said Grill stopped by Kerner's grandmother's house for an alleged drug deal that happened in the garage of the home.

The witness said he was told by Kerner that Grill allegedly tried to rob him and Kerner shot Grill and then "beat him with a pipe wrench until he died," the charging document states.

Kerner then made Lanham, who had been waiting in a car, come into the garage and shot her in the head after telling the teen she could leave, the witness told detectives.

Molley Lanham, 19, and Thomas Grill Jr., 18, went missing on Feb. 25, 2019. St. John Police Department

Kerner then put the bodies into the trunk of the car, drove to a wooded area and set it on fire with the bodies still inside, according to prosecutors. The witness told detectives Kerner said he planned on getting a U-Haul, putting the car inside and then dumping the vehicle into the Mississippi River, according to the charging document.

The witness told investigators that Kerner allegedly said, "he had done this before and knew how to cover up a murder and get away with it," according to the charging document.

The witness also told detectives that Kerner allegedly threatened to kill them and their family if they went to police.

The Porter County Sheriff's Office said they believe the bodies found in the Honda are Grill and Lanham, but have not yet confirmed that.

Kerner was arrested Saturday and is being held without bond, authorities said.

Grill and Lanham were both recent graduates of Hanover Central High School.

"This is a heartbreaking loss for HCSC and the entire community," the district said in a statement to NBC News. "Molley was involved in soccer and track. Thomas was involved in the building trades. They will be sorely missed."