A 21-year-old American citizen has been arrested for the fatal shooting of a US Border Patrol Agent in Vermont this week.

Teresa Youngblut, believed to be from Washington state, has been charged in federal court with using a deadly weapon while assaulting the border patrol agent and using and discharging a deadly weapon in that assault, the US Attorney’s Office for the District of Vermont announced.

According to the statement, federal law enforcement had been observing Youngblut and male citizen of Germany, identified by CNN as Felix Baukholt, for a few days after hotel employee in Lyndonville reported seeing them wearing tactical clothing and protective gear while carrying weapons. A few hours before the shooting January 20, law enforcement saw the pair in a Walmart parking lot in Newpoert, where Baukholt was seen wrapping things in aluminum foil.

Border patrol agents initiated a traffic stop of a Toyota Prius — registered to Baukholt, who was traveling on an expired visa — on Interstate 91 in Coventry.

“During the stop, Youngblut fired her handgun without warning toward at least one of the Border Patrol Agents while outside the vehicle, the US Attorney’s Office said. “Her German companion also tried to draw a firearm, and at least one Border Patrol Agent fired his service weapon.”

In the exchange of gunfire, Border Patrol Agent David Maland was fatally wounded, and Baukholt was killed. Youngblut was also shot and was taken to a hospital for treatment.

According to CNN, agents found multiple firearms, ammunition, two-way radios, and about a dozen other electronic devices in a search of the vehicle, along with military grade gear that included a ballistic helmet, night-vision goggles, and a tactical belt with a holster.

Additionally, court documents said, they also found a journal that appeared to belong to Youngblut and identification documents, and utility, lease, and travel information connected with multiple states.

Investigators have not speculated about what Youngblut and Baukholt may have been planning, if anything.