For two days, an assault-style rifle was moving around Montgomery County.

It had been stolen from the trunk of a Rockville police car, authorities said, before being recovered during an investigation of a stolen car on school property being driven by an 18-year-old.

Mario Granado Alvarado of Silver Spring was arrested Monday and charged by Montgomery police with possession of a deadly weapon on school property, possession of an assault weapon, possession of a stolen vehicle and other firearm-related charges.

Alvarado, a junior at Albert Einstein High School in Kensington, was taken into custody near the school. Authorities announced his arrest Wednesday. A 15-year-old was arrested on auto-theft offenses.

School officials said the weapon found in a car parked near a football field at Einstein was never taken into the school building. Montgomery County Schools Superintendent Jack R. Smith sent a letter to parents and guardians about the investigation, saying the district “does not tolerate guns on school property.”

Mario G. Alvarado (Montgomery County Police Department)

He added that any student who brings guns will be referred to law enforcement and disciplined accordingly.

The AR-15 assault-style rifle and a shotgun were stolen from the unmarked police car April 29, when it was parked in a residential neighborhood in the Kensington area, authorities said.

Both of the police-issued weapons were secured in the trunk of the car, said Officer Rick Goodale, a Montgomery police spokesman. He added that there were no signs of forced entry but that someone gained access to the trunk.

The officer filed a police report on the stolen guns.

Two days later, at 10:33 a.m. Monday, a school resource officer noticed Alvarado walking into the school and carrying a black bag.

According to charging documents, Alvarado reappeared a few moments without the black bag. He then got into a blue Mazda that was parked at the school by the football field and started driving, authorities said. When the school resource officer put on his emergency lights and tried to make a traffic stop, Alvarado got out of the car and at one point ran before police caught up, police said in charging documents, and said to police: “I scared. I scared.”

Alvarado was arrested, and officials said a computer check of the Mazda’s registration revealed that the car was stolen April 27 from a house in Wheaton. In the trunk of the car, officers near the school found the stolen assault-style rifle as well as ammunition.

Police also searched a stolen black Nissan Frontier that was parked near Einstein High School and found a stolen shotgun, according to a news release. The truck had been stolen April 22 from a home in Wheaton.

On Tuesday, officials said the 15-year-old was released to the custody of his parents.

Alvarado was released on a $2,000 bond, said a district spokesman. The spokesman added that the judge released him with an order that he stay away from Einstein High School.