WATERVILLE — A Waterville man plotted with two other teens over Instagram to attack a mosque in Chicago and then move onto other mosques or synagogues until finally being shot by police, a recently unsealed court document says.

A declaration written by Federal Bureau of Investigation Special Agent Garrett Drew was sealed by the court for 30 days so as not to “jeopardize the ongoing investigation” being conducted by the federal agency.

FBI investigators found two hand-painted ISIS flags at the apartment of Xavier Pelkey, 18, of Waterville after arresting him in February and charging him in connection to a planned attack on a Chicago mosque. Court documents indicate Pelkey had homemade explosive devices that investigators believe he planned to use in an attack on a Shia mosque in Chicago. Taken from court documents

The document includes information regarding the apparent motives and inspiration of Waterville resident Xavier Pelkey, 18, in planning the attack. It says that two hand-painted ISIS flags were found in his apartment when he was arrested in February, along with three “homemade explosive devices.”

The declaration, which was unsealed Friday, details a plot Pelkey allegedly hatched with two other teens, one in Chicago and one in Kentucky, using Instagram and other chat services.

The Chicago teen, the first one arrested as part of the investigation, told federal agents he had been talking to someone known as “Abdullah” on Instagram about a plan to gather weapons and explosives and meet in Chicago. The plan was to then separate children from adults in a Shia mosque and murder all the adults, the document says. The teen told agents the Instagram account name of “Abdullah.”

“I know from my experience in counterterrorism investigations that adherents of Islamic extremist groups such as ISIS consider other Muslims, including the Shia sect of Islam, as non-Muslims and have historically targeted them in terrorist attacks,” Drew wrote in his declaration.

If they were not apprehended by police after the attack, the three individuals planned to move on to another Shia mosque or Jewish synagogue to execute the same plan until they were shot by police, the Chicago teen told investigators.

The Chicago teen was found with a Remington pump shotgun, swords, knives, a bow and arrows, multiple homemade ISIS flags and multiple electronic devices when he was arrested in early February, the document says.

On Feb. 8, the FBI served Meta Inc., Facebook’s parent company, with an “emergency disclosure” to obtain any IP addresses associated with the Instagram account of the person identified as “Abdullah.” With the IP addresses in hand, the FBI then issued another emergency disclosure to Charter Communications and discovered that the addresses were linked to an individual in an apartment at 80 Front St. in Waterville, the document says.

Pelkey was arrested at his 80 Front St. apartment on Feb. 11 and confirmed he owned the Instagram account belonging to “Abdullah.”

Federal agents found three homemade explosive devices that consisted of fireworks taped together, laced with staples, pins and thumbtacks, that investigators believe were intended to serve as shrapnel when detonated.

A second juvenile, suspected of “conspiring to provide material support to a foreign terrorist organization,” was arrested in Kentucky a few days later as part of the investigation, the document says.

The Kentucky teen told agents that “Abdullah” said he wanted Allah to make him a “shaheed” and die while fighting in the cause of Allah.

“I know from my training and experience that ‘shaheed’ is an Arabic word that means a martyr in Islam,” Drew wrote in the declaration.

Pelkey is facing a federal charge of possessing unregistered destructive devices, punishable by up to 10 years in prison, and a $10,000 fine. He will be held behind bars until his case goes to trial, according to Assistant U.S. Attorney Craig M. Wolff.

As of Thursday, a date for Pelkey’s trial had not been set.