A Washington state woman has been charged in the fatal shooting of a U.S. Border Patrol agent during a Vermont traffic stop that happened days after authorities began watching her and a German companion, who also died in the highway firefight, the FBI said Friday.

Teresa Youngblut, 21, faces two weapons charges in connection with the death of Border Patrol Agent David Maland, 44, who died Monday during the shootout in Coventry, a small town about 20 miles from the Canadian border.

According to an FBI affidavit, a border agent pulled over Youngblut and Felix Baukholt on Interstate 91 because Baukholt appeared to have an expired visa. Youngblut, who had been driving Baukholt’s car, got out and opened fire on Maland and other officers without warning, the FBI alleges. Baukholt tried to draw a gun but was shot, the affidavit states.

At least one border agent fired on Youngblut and Baukholt, but authorities haven’t specified whose bullets hit whom.

“The events leading to this prosecution tragically demonstrate how the men and women of law enforcement regularly put their lives on the line as they try to keep our communities and our country safe,” acting U.S. Attorney Michael Drescher said in a statement. “We intend to honor them, and the memory of Border Patrol Agent Maland, by performing our prosecutorial duties so that justice may be done.”

Investigators had been performing “periodic surveillance” of Youngblut and Baukholt since Jan. 14 after an employee at a hotel where they were staying reported concerns after seeing Youngblut carrying a gun and she and Baukholt wearing black tactical gear, according to the affidavit. Investigators tried to question the duo, who said they were in the area looking to buy property but declined to have an extended conversation, the FBI said.