Federal authorities on Tuesday charged a demonstrator from Willow Grove in connection with the New Year’s Eve vandalism of the U.S. courthouse and prison in Center City.

Josie Robotin, 25, had previously been arrested by Philadelphia Police for attempted arson after officers said they discovered a Molotov cocktail, several firecrackers, lighters, and a small plastic container labeled “fire starter” in a backpack she had been carrying.

But the new federal case on a lone charge of possessing an unregistered firearm — in this case the homemade explosive — threatens to increase the potential prison sentence to 10 years should Robotin be convicted.

It’s also in keeping with the hard stance the U.S. Attorney’s Office has taken toward demonstrators it says put public safety at risk amid protests that proliferated across the city over the last year.

“Homemade Molotov cocktails can be incredibly dangerous,” Acting U.S. Attorney Jennifer Arbittier Williams said in a statement. “Here, [Robotin] is alleged to have carried the materials needed to endanger the lives of many people.”

Robotin’s attorney, Marni Jo Snyder, declined to comment. Her client was released pending trial from the same prison she’s accused of vandalizing after a brief initial court appearance Tuesday afternoon.

» READ MORE: Organized anarchists? Homegrown rabble-rousers? Philly police say they aren’t sure what motivated vandals.

In court filings, FBI agents described a chaotic scene that erupted outside the Federal Detention Center at Seventh and Arch Streets around 8:30 p.m. on Dec. 31.

Forty to 50 people dressed in black gathered near the building, setting off fireworks and spray-painting slogans such as “ACAB” — an abbreviation for “All Cops Are Bastards” — on the facade.

As the group spread out, some smashed a Philadelphia Sheriff’s Office vehicle parked at Seventh and Market Streets with a hammer, while others moved toward the Robert C. Nix U.S. Courthouse near the same intersection, agents said.

There, the crowd graffitied the courthouse’s exterior and threw bricks through its windowed entryway, before scattering when police arrived on the scene.

Officers later detained and charged seven people, including Robotin, who they stopped several blocks away, covered in spray paint and walking with two women — one of whom was carrying a gas mask.

They told officers they were coming back from dinner and were unaware of the earlier incident at the federal buildings, according to court documents.

Federal prosecutors didn’t say Tuesday whether they were also pursuing charges against the six others arrested that day.

So far, though, the FBI has focused its protest-related investigations on individuals suspected of setting fires, torching police cars, or bombing ATMs in incidents across the city.

The six others facing state charges in connection with the New Year’s Eve vandalism are Meredith Tooker, 26; Adam McVicker, 25, of Coatesville; Dustin Callahan, 24, of Farmingdale, N.J.; Allison Donohue 23, of Wallingford; Josey Augustine, 31, of Royersford, and Sydney Miller, 22, of Philadelphia.

All have been released on bail and await preliminary court hearings in their cases.