EVERETT, Wash. — Dakota Reed’s mind brimmed with thoughts of mass murder. In November, he wrote on Facebook, “I am shooting for 30 Jews.”

The next month, he uploaded a video of himself in his bedroom of his mother’s Seattle-area home proudly displaying new gun sights he had mounted on his AR-15-style semiautomatic rifle. White supremacist propaganda adorned the walls. He said he was “fixing to shoot up” a school.

The F.B.I., which had been investigating the 20-year-old Mr. Reed for about four months, weighed charging him. But federal prosecutors were concerned that the threat was too vague, so the F.B.I. quickly passed the case on to local law enforcement officials, who thought they could build a case under state law. In early December, a detective from the Snohomish County Sheriff’s Office arrested Mr. Reed. He pleaded guilty in May to making bomb threats and was sentenced on Tuesday to a year in jail.

The outcome was typical of the limits the F.B.I. faces investigating domestic terrorism cases, roughly defined as violent acts inside the United States intended to intimidate a part of the population. The First Amendment protects hate speech and other activities that might be early indicators of plans to commit violence, keeping agents from investigating or making arrests in many cases. Agents cannot always rely on federal law, unlike in so-called international terrorism cases where statutes were enacted to address the threat after the Sept. 11 attacks. Instead, the F.B.I. often turns to local prosecutors to charge people they are concerned might be planning domestic attacks.