(NewsNation) — An Idaho 18-year-old was arrested after he allegedly made plans to carry out attacks targeting local churches in the name of ISIS, the U.S. Department of Justice said.

Alexander Scott Mercurio, who pledged his allegiance to ISIS, was charged with “attempting to provide material support or resources to a designated foreign terrorist organization,” according to the Justice Department.

He was arrested a day before he allegedly planned to attack churches in Coeur d’Alene on Sunday “using weapons, including knives, firearms, and fire,” according to the affidavit cited by the Justice Department.

Investigators said the attacks were planned to coincide with the end of the Muslim holy month of Ramadan, NBC News reported, and Mercurio allegedly wrote a message to an FBI source detailing his plan.

“Stop close by the church, equip the weapon(s) and storm the temple, kill as many people as possible before they inevitably disperse/scatter, then burn the temple to the ground and flee the scene,” Mercurio allegedly wrote, according to court documents cited by NBC News.

According to prosecutors, Mercurio said he’d repeat this for all 21 churches in town, NBC News reported.

The FBI’s Joint Terrorism Task Force conducted the investigation.

“This case should be an eye-opener to the dangers of self-radicalization, which is a real threat to our communities,” said Shohini Sinha, special agent in charge of the Salt Lake City FBI.

 If convicted, Mercurio faces a maximum penalty of 20 years in federal prison, according to the Justice Department. He’s currently in custody awaiting his initial court appearance.