A federal grand jury indicted a father and son in the kidnapping of two U.S. Forest Service employees who were held hostage in Northern California, and the father faces new charges related to the 15-hour standoff, officials said Thursday.

Joseph Charles Henrichsen, 49, and his son, Phoenix Henrichsen, 23, had already been charged with two counts each of kidnapping a federal employee, the U.S. Attorney’s Office, Eastern District of California, announced in a news release.

Joseph Henrichsen now faces additional charges of assaulting a federal employee with a deadly weapon, brandishing a firearm during a crime of violence and assault on a federal officer, the release said.

The alleged kidnapping took place on July 16 in Shasta-Trinity National Forest, which is roughly roughly 115 miles south of the Oregon border and 150 miles north of Sacramento, according to officials.

Officials have not named the two U.S. Forest Service employees, who were biologists, and have not said if they’ve identified a motive for the kidnapping.

The two U.S. Forest Service employees were conducting routine environmental work near the forest’s Gumboot Lake Campground when Joseph Henrichsen approached them with an AR-10 semiautomatic rifle, officials said.

The elder Henrichsen bound their hands, and with his son’s help, forced them into a remote trailer where they were held for the next 15 hours, the release states.

The younger Henrichsen called multiple individuals stating that he had taken two federal employees hostage and was armed. In one of the calls, he said he had “live rounds ready to [expletive] anyone who [expletives] with me,” officials said in earlier news release.

Lawyers for the Henrichsens did not immediately return requests for comment.

Speaking at a press conference in the days after the kidnapping, Siskiyou County Sheriff Jeremiah LaRue and other officials laid out a timeline of the kidnapping.

Authorities received a 10:55 a.m. call from a U.S. Forest Service law enforcement officer reporting that the employees were zip-tied and being held at gunpoint inside a trailer at Gumboot Lake, near Mount Shasta, LaRue said.

LaRue said his office sent deputies to the location — which he described as a rural and rugged area — around noon and deployed drones. The FBI then deployed agents from Redding, a crisis negotiation team and SWAT, as well as a hostage rescue team and assets from Quantico, Virginia, according to officials.

Just after 1 p.m., drones helped authorities identify a trailer where Henrichsen was, according to officials. Soon after, local, state and federal authorities began to descend on the scene, according to officials.

Negotiations started at 4 p.m. and took hours before FBI crisis negotiators secured the safe release of both victims and the surrender of both suspects, officials said.

Benjamin Pratt, senior deputy prosecuting attorney for Whatcom County, told NBC News in a previous interview that he unsuccessfully tried to prosecute Joseph Henrichsen after a 2022 harassment case involving his neighbor.

The elder Henrichsen was accused of harassing and targeting a neighbor he thought to be Russian around the time Russia first invaded Ukraine. Public records state that Henrichsen engaged in threatening behavior toward the neighbor, including discharging a firearm, setting off fireworks, leaving zip-tie restraints on the property, and posting a note with an explicit drawing that said “Assassinate Vlad Putin.”

He was charged with a hate crime, malicious harassment and felony harassment in connection with the incident, according to the records.

The case was dropped after the state was not able to provide treatment in a timely manner due to a capacity issue, according to court documents. The Washington State Department of Social and Health Services declined his form to expedite the defendant’s treatment, Pratt said.

The Department of Social and Health Services previously told NBC News there was a delay in competency services in 2022 “due to a lack of bed space.”

The 2022 incident was far from Joseph Henrichsen’s first run in the with law. Since 1998, he has been charged with a long list of crimes, including assault with a deadly weapon, brandishing a weapon and attempted robbery, according to public records.

Both Henrichsens are in custody pending trial. If convicted on the kidnapping charges, both men face a maximum statutory penalty of life in prison and a $250,000 fine, according to officials. If convicted on the additional charges, Joseph Henrichsen faces a maximum sentence of 20 years.