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Washington — An 18-year-old from Idaho was arrested Saturday and accused of plotting to kill churchgoers in his town in the name of ISIS, according to court documents unsealed Monday. 

Alexander Mercurio was charged in a criminal complaint with providing material support to a foreign terrorist organization. He remains in custody and has not entered a plea, according to court records.  

Investigators alleged Mercurio was about to attack at least one church in his area on April 7 — a Sunday — with guns, knives, and flammable chemicals, but they said they foiled his plans before he could carry out his plan.

Mercurio is accused of writing to an unnamed FBI source that he was set to “stop close by the church, equip the weapon(s) and storm the temple, killing as many people as possible.” His goal, according to investigators, was to carry out an act of martyrdom before the end of Ramadan — the holy month observed peacefully by Islam practitioners across the globe that emphasizes prayer and fasting — and pledge his allegiance to the leader of ISIS.

“I am going to perform a martyrdom operation very soon,” he allegedly wrote in one recent message. “The targets will be the various churches in my town.”

In the newly-released court documents, prosecutors described an evolution of beliefs beginning in 2022 when messages and posts allegedly revealed the teenager’s interest in ISIS and his parents’ apparent dissatisfaction with his religious beliefs. He stored media on his electronic devices including a jihadi chant, investigators said, and posted pro-ISIS material online. 

Court documents revealed Mercurio also allegedly told members of a terror-affiliated group chat that he had previously been in contact with an individual who was later arrested for providing financial support to the terrorist group. 

Earlier this year, he met at various points with a confidential FBI source and reiterated his desire to die a martyr. In meetings and via online messaging, Mercurio allegedly revealed that he planned to “donate every last cent in my bank to the State (ISIS)” and wanted to film a video message in support of ISIS before he carried out his attack. 

“I really want to (carry out) an operation,” investigators alleged he said recently, and at a different time told the confidential FBI source that he wanted to “fix” what he saw as too many churches in his area.

According to prosecutors, his plan involved attacking his father with a metal pipe, stealing his guns and using them in the attack. Other components allegedly included using chemicals like hand sanitizer or butane to destroy the churches he targeted.

Charging documents alleged that on April 3, Mercurio declared his allegiance to the leader of ISIS, holding a knife and taking a photo in front of a flag wielded by the terrorist organization.  

When asked by the FBI confidential source on April 5 if he was going to postpone his plans, prosecutors said Mercurio responded that he wasn’t, as described in court records. 

The FBI executed a search warrant at his family’s home and arrested Mercurio on Saturday, one day before they said he intended to carry out the attack. Investigators seized a metal pipe, butane, hand sanitizer, a machete and multiple firearms locked in his father’s closet. 

“The defendant swore an oath of loyalty to ISIS and planned to wage an attack in its name on churches in Coeur d’Alene, Idaho,” Attorney General Merrick Garland said in a statement announcing Mercruio’s arrest on Monday. “Thanks to the investigative efforts of the FBI, the defendant was taken into custody before he could act.” 

An attorney for the defendant could not be immediately identified.

The charges come just days after federal law enforcement agencies including the FBI and the Department of Homeland Security issued a bulletin to law enforcement across the country warning of “possible threats to public gatherings in the United States from threat actors” inspired by last month’s  deadly terror attack on a Russian concert hall. ISIS-K claimed responsibility for the massacre that left over 100 dead. 

Last week’s law enforcement bulletin obtained by CBS News did not offer specifics but said “ISIS-aligned media groups released messaging encouraging attacks against other mass gatherings.”