A 17-year-old Utah boy is accused of killing his mother and 8-year-old half-brother after prosecutors say a masked figure matching his build was caught on camera roaming the neighborhood in the hours after a late-night shooting.

Saratoga Springs police arrested the teen on Aug. 14 after a 17-month investigation involving dozens of search warrants, hundreds of pieces of evidence and extensive forensic testing, the department said.

The teen was booked into juvenile detention after detectives screened two counts of first-degree murder with the Utah County Attorney’s Office, police said.

Prosecutors later charged him in juvenile court with two counts of aggravated murder and obstruction of justice, according to charging documents obtained by home-surveillance video allegedly showed a masked figure walking through the neighborhood between 2:34 a.m. and 2:59 a.m. on March 28.

Prosecutors said the person was consistent with the teen’s height and build, and that the mask and some clothing appeared consistent with items he had worn. The person’s route and gaps in the video suggested the weapon could have been discarded in several places, including the nearby Jordan River, according to the charging documents.

A warrant unsealed last year detailed a search of the teen’s room, where detectives allegedly found handwritten notes mentioning "serology," "DNA" and "firearm testing." Another note read, "This is a murder story," KSL reported.

Police said the home was immediately secured as a crime scene and investigators began the extensive work of documenting the scene, collecting evidence, conducting interviews and pursuing leads.

Over the next 17 months, detectives executed dozens of search warrants, collected hundreds of items of evidence and reviewed extensive video, records and other information, police said.

Numerous pieces of evidence were submitted to forensic laboratories, and some examinations required significant time before investigators could take additional steps, according to the department.

"As additional evidence and forensic results became available, detectives continued to evaluate and corroborate the information developed throughout the investigation," Saratoga Springs Police Chief Andrew Burton said following the arrest.

"Most recently, the examination of additional evidence, combined with the totality of the investigative work conducted over the past 17 months, placed our detectives in a position to identify a suspect and establish probable cause for an arrest."

The FBI, Utah State Crime Laboratory, West Jordan Police Department and Utah County Sheriff’s Office Evidence Team assisted in the case, police said.

The department said its work is not finished, and that investigators continue to conduct follow-up interviews and analyze additional information.

The arrest came after Lyman and Eli’s relatives spent more than a year publicly pleading for answers.

"Not a day has passed without feeling the weight of this loss. Our family is forever changed," the Orton family said in a March statement released as the one-year anniversary of the killings approached.

"We miss Jessica and Eli every moment — their laughter, their light, and the love they brought into our lives."

The family said at the time that it knew evidence was still being processed and urged anyone with information to come forward.

Following the arrest, the Orton family said it was grateful to the agencies that worked on the case but stressed that no prosecution could undo the loss.

"Today’s arrest marks an important step toward justice for our daughter and sister, Jessica Orton Lyman, and our grandson and nephew, Eli Painter," the family said in a statement released through police.

The family said it expects the person responsible to be held "fully accountable for their actions, without excuse, exception, or leniency."

Jason Painter, a family member of both victims, said Lyman was "a devoted mother whose strength and love touched everyone around her," while Eli was "a bright, loving little man whose laughter and curiosity brought joy to everyone who knew him."

"They were so much more than the tragedy that took them," Painter said. "Their lives were filled with love, and they deserved many more years together."

The families said they will continue to honor Jessica and Eli by remembering "the love they shared, the lives they lived, and the legacy they leave behind, not just how they were taken."

Fox News Digital reached out to the Utah County Attorney’s Office for additional comment.