An American Muslim convert allegedly bought illegal grenades from an undercover FBI agent and was stockpiling ammunition in an underground container, according to federal court records unsealed Monday in Detroit.

Interviews found in court records portray Sebastian Gregerson, aka Abdurrahman Bin Mikaayl, as a "homegrown violent extremist" who amassed an "arsenal" to carry out an attack while cheering mass killings in Paris and Orlando, the Detroit News reported.

Both the FBI and the Detroit newspaper used the language of the Obama administration, referring repeatedly to Gregerson as a "home-grown violent extremist" who seemingly just randomly fell into a state of allegiance to the Islamic State, also called ISIS.

Near the end of the Detroit News article the newspaper finally gets around to telling its readers that Gregerson, 29, converted to Islam after high school and worked as an employee of the Minneapolis-based Target store chain.

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The FBI obtained a search warrant July 31 listing probable cause to search Gregerson’s home and vehicle following 16 months of taped discussions Gregerson had with an undercover FBI agent who he thought was a fellow ISIS supporter. Agents were reportedly looking for evidence Gregerson was providing material support to ISIS, a designated foreign terrorist organization.

They raided Gregerson’s Detroit home after he allegedly bought five illegal grenades that same day from the undercover FBI agent.

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The search netted seven rifles, two AK-47s, a shotgun, handguns, thousands of rounds of ammunition, computer equipment and cellphones, according to a search warrant inventory obtained by the Detroit News. There were also grenades and grenade launchers.

The weapons cache also included tactical knives resembling those used by "extremists" to behead Western hostages such as James Foley and the 21 Coptic Christians on a beach last year in Libya, according to the FBI.

Gregerson also owned an underground ammunition storage container that the FBI said he buried in his back yard.

Agents had permission from a federal judge to dig up Gregerson’s yard and use a metal detector in hopes of finding the buried container, according to the search warrant affidavit.

Agents claim to have not found enough evidence to prove that Gregerson was providing material support to ISIS or any other designated foreign terrorist organization. He has not been charged with a terrorism-related crime, the News reports, but the investigation is not over. The FBI found no evidence of a specific planned attack, or if they did it was not revealed in the court documents, but he could face up to 10 years prison on the weapons charges.

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The charges include unregistered possession of a destructive device and unlicensed receipt of explosive materials.

Gregerson met with the undercover agent at a Dearborn park in June, according to court records. He reportedly pointed out a park ranger and boasted about how easy it would be to ambush him.

“People like this, this isn’t even a challenge,” Gregerson allegedly told the agent. “You know they talk about all these lone wolves and all this kind of stuff, you know, threats. Listen, if someone really wanted to do something that knew what they were doing, they could do something, and they could do a lot.”

The secretly taped conversations captured Gregerson talking about the Islamic State and weapons, particularly grenades and grenade launchers, according to the affidavit, the News reported.

Gregerson expressed his interest in committing violent acts and celebrated rise of the Islamic State in Iraq and Syria, according to the FBI.

“Gregerson told the (undercover agent) that he watched al-Baghdadi’s speech some five times that day and celebrated the event with associates,” according to the affidavit. “Gregerson stated: ‘We could not stop smiling.’ ”

He also talked about threatening local Islamic leaders.

“There is no more ambiguity about this case,” Seamus Hughes, deputy director of George Washington University's Program on Extremism, told the News. “He’s been a true believer for a few years.”

Praise for Orlando attack

The FBI also said Gregerson praised Islamic State-inspired extremists, including Omar Mateen, the American-born son of Afghan immigrants who killed 49 people at an LGBTQ nightclub in Orlando on June 12.

Gregerson referred to Mateen as his “brother” and offered advice on how Mateen could have boosted the body count, according to court records reported on by the News.

“If he would have just kind of stayed in there and held them off for a few more hours ... we would’ve been talking about 75 (fatalities) ...” Gregerson reportedly told the undercover FBI agent during a recorded conversation on June 21. “Then maybe he could have waited for the cops to come inside and try to pick off a couple of them on the inside ...”

During the prolonged investigation, Gregerson repeatedly expressed his support for Islamic State "extremists," court records allege. Gregerson frequently visited Islamic State websites and praised the extremists’ violent methods, according to the FBI.

Gregerson said he wanted to move with his wife and four-year-old twin sons to Islamic State territory and “remain there permanently,” according to the search warrant affidavit.

Until Monday, the FBI and U.S. Attorney’s Office had refused to disclose any evidence linking Gregerson with Islamic State extremists.

The FBI issued a statement on the case Monday.

“As a pending investigative matter yet to be adjudicated, the FBI cannot comment on the Gregerson investigation,” said David Gelios, special agent in charge of the Detroit office. “That said, the FBI prioritizes the investigation of terrorist organizations and the homegrown violent extremists that they inspire. Terrorism is something that requires constant vigilance on the part of the public and routine coordination with all of our public and private partners.”

David Tholen, Gregerson’s court-appointed lawyer, said during a recent court hearing that Gregerson is a gun enthusiast, hunter and survivalist who has no criminal record, the News reported. The firearms in question were bought legally, Tholen said.

Raid yielded al-Awlaki CDs

During the raid of Gregerson’s home on July 31, agents found CDs marked “Anwar al-Awlaki,” the al-Qaida recruiter who met with Umar Farouk Abdulmutallab before the failed Christmas Day 2009 terror attack on a Detroit-bound plane, according to the search warrant records. Al-Awlaki was killed in a 2011 drone strike in Yemen.

Gregerson claimed to have all of al-Awlaki’s lectures, which have been used to radicalize homegrown violent extremists, according to the FBI.

In November, after what the Detroit News referred to as "extremists" killed 130 people in a series of Islamic terrorist attacks on nightclubs and the national stadium in Paris, Gregerson voiced his support for the attacks on Facebook.

"Only a fool would believe they are untouchable and that just because you kill people a thousand miles away every day it will not affect you,” Gregerson allegedly wrote. “The world is at war, when one engages your enemy in their homeland your homeland is not exempt. Accept responsibility for your actions if you dont (sic) want war in your land dont (sic) bring it to someone else’s home."

Gregerson also distributed the Islamic State’s official English language magazine, Dabiq, according to court records.