Insets, left to right: Bobby Jackson Jr. and Rebecca Rivera (Blackfoot Police Department). Background: Trailer park in Blackfoot, Idaho, where Jackson allegedly killed Rivera (Google Maps).

A 16-year-old Idaho boy is accused of stealing a gun from his workplace and then shooting a woman to death because he said she wouldn't give him a ride and he thought that her brothers had a role in his friend's death.

"I put a bullet in her brain," the suspect Bobby Grant Jackson Jr. reportedly told cops, referring to the victim, 30-year-old Rebecca Rivera.

Jackson is now facing a first-degree murder charge in the shooting death that occurred Thursday in Blackfoot, some 250 miles east of Boise. According to police, family members found Rivera dead from a gunshot wound around 4 p.m. at her mobile home in the 1100 block of Broadway Street.

A probable cause arrest affidavit obtained by Law&Crime said Jackson was working earlier that day for a subcontractor renovating a hospital. An employee told investigators that Jackson had asked to go to the bathroom and was gone a long time. The man went looking for Jackson and realized that a handgun he had inside a lunch box in his truck was missing and Jackson was nowhere to be found. The employee called police.

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Officers obtained surveillance footage that showed Jackson enter the truck and grab something. Investigators learned that Jackson had been living at a halfway house while on probation from a previous arrest for armed robbery. An employee at the halfway house told cops Jackson's phone showed he was at a gas station in the 900 block of Broadway Street, which is just down the street from the murder scene.

Detectives spoke with Rivera's neighbors who said they saw Jackson earlier in the day asking for a ride. Surveillance images confirmed he was at the trailer park sometime after 12:40 p.m. Investigators also found that Rivera was murdered with a 9 mm handgun — the same type of weapon that Jackson allegedly stole from the worksite.

An arrest warrant was initially obtained for second-degree murder, and cops took Jackson into custody Friday at a home in Pocatello. Cops say Jackson had "no known relationship" with Rivera. Local news website EastIdahoNews.com obtained an amended complaint that contained Jackson's interview with detectives.

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Investigators reportedly wrote that Jackson asked Rivera for a ride and when she declined, he said "f— it" and went into the trailer and killed her. He also claimed that he recognized Rivera and "believed her brothers had killed his friend," the affidavit reportedly said. When asked if he killed Rivera as payback, he reportedly answered in the affirmative.

"I put a bullet in her brain," he allegedly said.

After his interview, prosecutors upgraded the charge to first-degree murder. He's being charged as an adult.

Jackson is being held at the 3B Detention Facility on a $1 million bond. His next court date is slated for Nov. 6.