Justin Olsen, 18, was the moderator of an online chat where he made light of mass shootings and made the threats against all federal agents, the nonprofit, and an undisclosed gay bar, according to a police report from the Boardman Police Department.
After searching his father's house in Boardman, a suburb south of Youngstown, FBI agents seized 15 rifles and 10 semi-automatic pistols, according to a criminal complaint. Investigators also observed an estimated 10,000 rounds of ammunition in one room and another 300 rounds of ammunition on the stairway leading to the second floor, according to the complaint.
ABC News could not immediately confirm who owned them, or if they were legally purchased.
When Olsen turned 18 in May, he boasted in a post that he couldn't “wait to start stockpiling weapons,” according to the police report.
He also wrote that he was planning on buying an AR-15 parts kit after posting a photo of it, the police report said.
The case began in February, when the FBI received a complaint about Olsen in Anchorage, Alaska, and traced his computer information back to Olsen's address.
On Aug. 6, an FBI agent approached a Boardman police detective with the information and “in light of the recent mass shootings in the United States," officials decided to arrest him. He was taken into custody the next day at his father's residence.
In the span of a week ahead of Olsen's arrest, 34 people were killed in three recent but separate mass shootings in El Paso, Texas (Aug. 3); Dayton, Ohio (Aug. 4); and Gilroy, California (July 28).
Olsen, who is charged with aggravated menacing, admitted to making the threats, according to police, but said it was all a joke and for fun.
He is currently being held at the Mahoning County Jail. It was not immediately clear if he had legal representation.
ABC News' Jack Date contributed to this report.