National Weather Service offices around the country are on guard after recent threats to agency infrastructure — specifically Doppler weather radars — from a violent militia-style group, emails from the National Oceanic and Atmospheric Administration’s security office show.

The group Veterans on Patrol, which the Southern Poverty Law Center defines as an anti-government militia organization, views the NWS’ network of Doppler radars as “weather weapons,” according to an internal NOAA email sent Monday and seen by CNN.

A previous email informing the NWS workforce of general, non-specific threats was sent May 1. Monday’s email was more action-oriented, with the NOAA security office noting they are aware of “several encounters,” physically or virtually, with Veterans on Patrol.

“This group is advocating for anyone and everyone to join them in conducting penetration drills on NEXRAD sites to identify weaknesses which can be used to ultimately destroy the sites,” the email stated, using an acronym for the weather radar network.

“The group referred to the NEXRAD system towers as ‘weather weapons,’ and claimed there were no laws preventing American citizens from destroying the ‘weapons,’” the email states.

It’s unclear what the group believes the radars are doing. The NEXRAD, or “next generation radar,” network has been in place since the 1990s and detects precipitation in the atmosphere. It can also help pinpoint tornadoes and severe thunderstorms, prompting timely, life-saving warnings. The system is also used by the Federal Aviation Administration and the US Air Force.

Among other actions, NOAA’s security office advises that the NWS implement a buddy system when working at remote sites and be alert for suspicious activity.

“Do not engage with anyone suspicious; instead, notify local law enforcement,” Monday’s email states.

The NOAA emails indicate the FBI and other law enforcement authorities are aware of the militia-style group’s activities and are tracking them, and have issued bulletins about the NEXRAD threat. CNN could not independently confirm the FBI’s involvement.

The security threat comes at a time of turmoil for the NWS, with many offices short-staffed due to hundreds of Trump administration layoffs and early retirements.

“Nobody likes federal employees being demeaned by the administration, but when it escalates to violence or threats of it, it starts to become really demoralizing,” said one NWS employee who requested anonymity because they were not authorized to speak to the press.

“We obviously deal with some fringe folks every now and then (chemtrails mostly), but specific threats to target facilities is pretty rare,” the NWS staff member said. “Given the general temperature of the country right now, this is stuff is especially concerning.”

Now would be a particularly inauspicious time for NWS radars to fail, whether due to mechanical issues or deliberate acts, since the agency has more than 90 vacancies for the technicians that normally repair them. That number is unlikely to change soon, since a hiring freeze is in place at the agency.