Federal prosecutors announced Friday that they have charged Teresa Youngblut with assault with a firearm in a shooting that killed a Border Patrol agent earlier in the week in Vermont.

Agents had stopped a Prius on Monday after receiving tips about suspicious behavior and believing the driver may have been an illegal immigrant.

Ms. Youngblut exchanged fire with the agents, killing Agent David Maland. The German man driving the Prius, Felix Baukholt, was also killed in the exchange and Ms. Youngblut was wounded.

“The senseless and tragic killing of a United States Border Patrol agent is a stark reminder of the immense sacrifices law enforcement officers make to protect our nation,” said James M. Ferguson, special agent in charge of the Boston office of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The FBI, in an affidavit, said Ms. Youngblut and Baukholt had drawn attention for their behavior in Vermont days ahead of the incident.

A hotel employee had first flagged them because they were dressed in tactical clothing and she was carrying a gun in a holster. When state and federal authorities tried to talk to them they brushed off the encounter, saying they were in the area to buy property.

They were observed again in tactical clothes and Ms. Youngblut with a gun on Sunday, and on Monday they were spotted in the Prius outside a Walmart where Baukholt “was seen wrapping unidentifiable objects with aluminum foil while seated in the vehicle,” prosecutors said.

The FBI said in an affidavit that Ms. Youngblut fired first, unprovoked. Baukholt then tried to draw his gun during the shootout.

Authorities recovered two pistols, three ammunition magazines, spent casings and intact cartridges from the scene that they said belonged to the Prius occupants. The FBI also found a ballistic helmet, a night vision monocular, full-face respirators, two-way radios and shooting range targets.

• Stephen Dinan can be reached at sdinan@washingtontimes.com.