In February, F.B.I. agents in Alaska who were researching an online chat room found that someone had posted several times about supporting mass shootings and assaults, and targeted Planned Parenthood, the authorities said.

The person’s online following was growing, they said. Delving deeper, the authorities found that he had written in June about shooting federal agents. Investigators traced his email address and found that it originated in Boardman, Ohio.

Last week, the authorities arrested Justin Olsen, 18, in Boardman, and he was charged on Monday with threatening to assault a federal law enforcement officer, according to documents filed in federal court. The authorities seized more than a dozen rifles, including some AR-15-style rifles, and about 10,000 rounds of ammunition from a home where Mr. Olsen was living, according the court filing.

It is not clear whether the weapons belonged to Mr. Olsen, or whether he even had access to them — many were kept in a “gun vault” in his father’s room, according to the filing.