The FBI said Friday that it arrested a Washington state woman in the fatal shooting of a U.S. Border Patrol agent in Vermont earlier this week.

Teresa Youngblut, 21, was charged in Monday’s killing of Border Patrol Agent David Maland, the FBI said, noting that Youngblut and a German man who died in the firefight that also killed Maland had been under surveillance for several days leading up to the confrontation.

Maland stopped Youngblut and Felix Baukholt on Interstate 91 in Coventry on Monday because Baukholt appeared to have an expired visa, according to an FBI affidavit. During the stop, Youngblut opened fired on Maland and other officers and 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.

Investigators had been performing “periodic surveillance” of the pair since Jan. 14 after an employee at a hotel where they were staying reported concerns about seeing Youngblut carrying a gun and both of them wearing all-black tactical gear. Investigators attempted to question them but the pair declined to have an extended conversation and said they were in the area looking to buy property.

About two hours before the shooting, investigators watched Baukholt exit a Walmart in Newport with two packages of aluminum foil. According to the affidavit, he was seen wrapping unidentifiable objects while seated in the passenger seat.

Authorities later found a ballistic helmet, night-vision goggles, respirators and ammunition in the car, along with a package of shooting range targets, some of which were used. They also found two-way radios, about a dozen “electronic devices,” travel and lodging information for multiple states and an apparent journal.