YAKIMA, Wash. — An 18-year-old is in custody after Yakima police said he was caught with the magazine for a .45-caliber semiautomatic pistol in his underwear.

The incident began at 8 p.m. Monday when a patrol officer made contact with the teenager while checking out reports of suspicious circumstances in the 1100 block of South Naches Avenue, according to a probable-cause affidavit filed in Yakima County Superior Court.

The teen is a convicted sex offender who had a felony warrant for his arrest for failing to register, the affidavit said.

That led to a search of his person, which in turn yielded a loaded magazine for a .45-caliber pistol in his underwear, the affidavit said.

In short order, officers located the actual gun, described in the affidavit as a Colt M1911A1, in a nearby parked car that belonged to the boy’s mother, the affidavit said.

Officers also found a photo on the boy’s cellphone showing him holding the pistol. He was lodged in the Yakima County jail on suspicion of first-degree unlawful possession of a firearm.