Last May, agents from the State Department’s Diplomatic Security Service showed up at an Orange County retirement community to serve a search warrant. They were joined by a man wearing a ballistic vest, a pistol strapped to his thigh and a badge on his chest, who introduced himself as a special agent with Homeland Security Investigations.

Donovan Nguyen accompanied the agents and was the first one through the door of the home they searched, one of the State Department agents recalled last month to a real agent with Homeland Security Investigations.

Nguyen, the agent wrote in an affidavit, is a civilian who has masqueraded as a federal agent for years, parading around in body armor, openly carrying firearms, pulling over motorists with red and blue lights installed in his pickup truck, and purchasing guns with fake Department of Homeland Security credentials.

Nguyen, who lives in the city of Orange, was arrested Monday and charged with impersonating a federal agent. It wasn’t immediately clear from court records if he had a lawyer.

Nguyen, 34, most recently worked as a security guard at Laguna Woods Village, a gated retirement community in Orange County, according to the affidavit.

David A. Prince, the special agent in charge of Homeland Security Investigations’ Los Angeles office, said agents spoke with Nguyen and his family on Monday and learned he had tried to get a job with several federal agencies, without success. Nguyen had wanted “to do something that would perhaps make his family proud,” Prince said in an interview.

Investigators served search warrants on several sites Monday and seized dozens of long guns and pistols, along with silencers, body armor, shields and police badges, Prince said. Nguyen had also used Homeland Security letterhead to acquire high-capacity magazines prohibited to the public, Prince said.

“This guy amassed an arsenal that could have outfitted a third of my office,” he said.

Saying his conduct “shocks the conscience,” Prince criticized Nguyen for threatening public safety and diminishing the agency’s credibility at a time when law enforcement officers are “under a level of scrutiny they’ve never experienced before in the United States.”

Jeffrey J. Gilgallon, the special agent in charge of Immigration and Customs Enforcement’s Office of Professional Responsibility in Los Angeles, said more impostor federal agents have cropped up in recent years, possibly because ICE has been more frequently in the news.

In 2018, a Fontana man was sentenced to two years in prison for impersonating a Homeland Security agent. Matthew Ryan Johnston admitted using red and blue lights installed in his car to pull over motorists, including one incident in which he gave chase, sirens flashing, and caused a traffic accident. Investigators found an arsenal of long guns in his home and a cache of explosives buried in the desert, court records show.

“They have this kind of wannabe mentality,” Gilgallon said in an interview. “Not only do they like impersonating law enforcement officers, they also like to be around them, and that’s usually their downfall.”

Federal agents were tipped to Nguyen in June by investigators from the Riverside County district attorney’s office, court records show.

The owner of a gun store in Riverside had reported a potential criminal case to the county prosecutor’s office and asked its investigators to partner with Nguyen. The gun salesman had sold Nguyen seven firearms over the years, with Nguyen presenting credentials at each purchase identifying him as a “lieutenant” with the Department of Homeland Security, the affidavit said.

Riverside investigators left Nguyen several voicemails, and in June, he called them back, according to the affidavit. Nguyen said he had reviewed the gun dealer’s tip but wasn’t willing to take the case because a witness had been deported, the affidavit said.

“His story really didn’t make sense,” Gilgallon said, and the veteran Riverside investigators “called our office and started asking questions.” No one by that name worked for the agency, investigators quickly determined, and they started digging into Nguyen’s background.

They found his LinkedIn profile, which describes him as a 12-year agent, and a YouTube interview in which he introduces himself as a special agent and discusses federal immigration policies.

Investigators learned Nguyen had previously worked for a contractor that provided security at a Department of Homeland Security base in Riverside, the Air and Marine Operations Center. One of Nguyen’s duties was to print access cards for employees and visitors, the affidavit said.

Nguyen was barred from the base in 2015 after an internal investigation found he had printed fake Homeland Security credentials for himself and two co-workers, according to the affidavit.

The Riverside gun salesman told investigators Nguyen and the co-workers used the ID cards to buy weapons. Presenting law enforcement credentials “allowed him to avoid taking and paying for certain firearm safety courses required by the State of California,” the affidavit said.

After being ousted from the Riverside base, Nguyen took a job as a supervisor at Laguna Woods Village, an upscale retirement community, where he worked alongside a number of retired police officers, the affidavit said.

Although the community’s security guards were supposed to be unarmed, Nguyen carried firearms openly and attributed the weaponry to his job as a federal agent, employees told investigators.

Tom Siviglia, a security manager and a retired Cypress police officer, said Nguyen often showed up to work late and left early, explaining he was “doing his ‘agent’ duties,” the affidavit said. He often came to the retirement community “tacted out” — wearing full tactical gear — and carrying a handgun, Siviglia told investigators.

Nguyen kept a plaque on his desk that identified him as a captain with the Department of Homeland Security, the affidavit said. He sent emails from a “dhs.gov” account he’d kept from his time working at the Riverside base, signing off emails with a signature that identified him as the “director” of an unspecified “JTF,” or joint task force, according to the affidavit.

With Carlos Rojas, the retirement community’s security director and Santa Ana’s former police chief, Nguyen discussed the civil unrest sweeping the country “in which DHS agents have played a prominent and controversial role,”the affidavit. Nguyen texted Rojas, “We got orders to shoot freely just now.”

Nguyen used red and blue lights installed in his Toyota Tacoma to pull over co-workers, the affidavit said. Robert Martinez, a retired police officer, told investigators he “was leaving work when he saw red and blue lights in his rear-view mirror and heard a siren chirp.” He pulled over and Nguyen pulled alongside him, the affidavit said.

“Did I scare you, Martinez?” he recalled Nguyen saying, before laughing and driving away.

Of all his alleged charades, the most daring came when a pair of agents from the State Department showed up at Laguna Woods Village to serve a warrant in May 2019.

Nguyen greeted the State Department agents in “tactical raid gear, a thigh holster with a weapon and ballistic vest displaying an HSI badge,” the affidavit said. Explaining he was assigned to a terrorism task force and had “worked all night,” Nguyen said he still wanted to help them serve the warrant, one of the agents recalled him saying.

He accompanied the State Department agents and was the first one through the door of the home they searched, the agents reported.

Prince, the Homeland Security official, said the public should be on guard for impostor agents; real federal agents don’t work alone — “we travel in twos or more” — and would rarely pull a motorist over, he said. “That’s a sign something is not right.”

If you suspect an agent is not who he says he is, Prince said, ask for his identification, take down his badge information, go to a safe location and call that agency’s office immediately.