A Tennessee man already awaiting trial for assaulting a police officer during the Jan. 6, 2021 riot at the U.S. Capitol was charged again Friday with plotting to kill the federal agents who investigated him and planning to attack the Federal Bureau of Investigation’s Knoxville, Tenn., field office.

The plan by Edward Kelley, 33 years old, was foiled this past week when a witness gave police an envelope containing what appeared to be a hit list bearing the names of 37 people involved in the investigation into Mr. Kelley’s alleged role in the Capitol assault, federal prosecutors wrote in newly unsealed court records.

The witness then cooperated with investigators, secretly recording phone calls with Mr. Kelley and another man, Austin Carter, 26, as they discussed plans to “take out” the agents at their Knoxville office, the records say.

“You don’t have time to train or coordinate, but every hit has to hurt, every hit has to hurt,” Mr. Kelley said in the calls, according to the documents charging him and Mr. Carter with conspiracy, retaliating against a federal officer, interstate communication of a threat and solicitation to commit a crime of violence.

Both men were detained after a Friday hearing in federal court in Knoxville, the Justice Department said. Lawyers for the men didn’t immediately respond to requests for comment.

Prosecutors said Mr. Kelley was wearing a gas mask and tactical helmet when he arrived at the Capitol on Jan. 6. They said he fought with a Capitol police officer on the west side of the building before using a piece of wood to break through a window near the Senate wing door, forcing his way into the building. Roughly 900 people have been charged in the attack.

Mr. Kelley was arrested on May 5 and released on his own recognizance.

Law enforcement began investigating the assassination plot on Tuesday after receiving the list, which included the names of agents who were involved a search of his home in May. The envelope also contained a thumb drive of footage from Mr. Kelley’s home security camera showing a law-enforcement officer approaching the house the day of his arrest.

The witness, an acquaintance, told the FBI in an interview that Mr. Carter had given him the envelope on behalf of Mr. Kelley, telling him to memorize it and “burn it when you’re done,” the court papers say. The witness told investigators Mr. Kelley had mentioned crafting the list earlier this month and asked him to “reach out to your cop buddies and see what information you can collect” on the targets.

On Wednesday, the witness recorded Mr. Kelley asking if he could stash some “weapons and ammo” at his house over the Christmas holiday, the papers said. In a later call, Mr. Kelley told the witness that if he didn’t hear from him within two days, it meant authorities were looking to arrest him and that the witness should recruit a group of people to attack the FBI’s Knoxville office.

The new case against Mr. Kelley comes as officials have cited an uptick in threats against law-enforcement officers in recent months. Threats involving federal agents specifically rose after the FBI’s August search of former President Donald Trump ‘s Mar-a-Lago estate in Florida. In one such case, Ricky Walter Shiffer Jr., who posted a call for violence after the search, came to the FBI’s Cincinnati office three days later armed with a rifle and tried to break into the security screening area before fleeing the scene and being killed in a gunfight with police.

Write to Sadie Gurman at sadie.gurman@wsj.com