“But,” she said, “that’s our new reality — it’s that anybody can be a target.”

In October, according to the court filing, Mr. Holzer told the undercover F.B.I. agent that he was going to scope the synagogue and sent the agent a video from outside. He then told the agent his plan was to poison members of the synagogue on Oct. 31, possibly with arsenic. He invited the agent to join him.

Later, Mr. Holzer and a friend identified in the documents as Skeeter met with three undercover F.B.I. agents, with whom he began to discuss alternate plots, including a plan to build Molotov cocktails and weld the temple doors shut. Mr. Holzer stated that he wanted to “vandalize the place beyond repair,” the documents said. He then visited the synagogue with the agents, where Mr. Holzer assessed that the Molotov cocktails would not be enough.

The agents “offered to supply” pipe bombs, according to the documents. Several days later, Mr. Holzer met the agents to pick up the bombs, which were inert, according to officials. Authorities arrested him shortly after, and he admitted that he had been planning to blow up a synagogue in the middle of that night.

Officials said he referred to the plan as “my mountain” and to Jews and the synagogue as a “cancer” to the community.

Mr. Holzer’s arrest marks the 13th time since the Pittsburgh attack that authorities have apprehended someone for allegedly plotting attacks or making threats against a Jewish community, according to a report from the Anti-Defamation League.

The organization had been tracking Mr. Holzer’s online activity since May 2016, according to Scott Levin, its regional director, and had repeatedly shared information with law enforcement, citing a concern that he might pose a threat to public safety.

Temple Emanuel is one of two synagogues in Pueblo, a city of about 112,000 people.

The area is divided politically and the temple is made up of people from across the political spectrum, Rabbi Becker said. She said the plot was particularly painful because Temple Emanuel had become a place where people with differing political views could talk “without a problem.”