A Syosset man suspected of posing as a federal air marshal was ordered held on $300,000 bail Friday after Nassau County police said they found him with an assault rifle in the backseat of his car, and five handguns and 8,300 rounds of ammunition in his home.

Police said they had gotten a tip and had Mark Vicars, 49, under surveillance when they made a traffic stop of his car on Jericho Turnpike at 7:40 a.m. Thursday.

Vicars activated the police-style emergency lights on the dashboard of his 2014 Dodge Durango as he pulled over, police said.

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A Daniel Defense .223 assault rifle, a loaded pistol, a knife, a tactical vest with three high-capacity magazines and body armor were found in the car, police said. He also had a windshield placard, often used to park in restricted zones, similar to those used by the Department of Homeland Security, police said.

Officers armed with a search warrant went to his home at 137 The Knoll, where they recovered the other handguns and “8,300 assorted rounds of assault ammunition,” Det. Lt. Richard LeBrun of the Nassau County Police Department said at a news conference in Mineola.

LeBrun said Vicars had no known terrorist connections.

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Vicars had no known arrest record, but LeBrun declined to say whether police knew other details of his background. “At this time we don’t see any immediate threat to the public,” he said.

The spokesman said the surveillance began after police got a tip that Vicars was posing as an air marshal. He declined to say when police got the tip.

Judge Joseph Girardi set the bail, cash or bond, at Vicars’ arraignment in First District Court in Hempstead, noting that the defendant faced up to 15 years in prison if convicted.

Vicars is charged with 17 counts of criminal possession of a weapon and possession of a forged instrument. He also was arraigned on drug possession charges for having medications for which he did not a prescription, according to court documents.

Police said Friday that the investigation is ongoing and urged anyone who has had contact with Vicars to call police at 800-244-8477.

With John Valenti