An arrest has been made in the fatal shooting of a U.S. Border Patrol agent during a traffic stop in Vermont, according to the FBI, as bizarre details of the suspects’ movements beforehand have been revealed in the arrest affidavit.

Teresa Youngblut, 21, of Washington state, was arrested on Friday and charged with assault on a federal law enforcement officer, according to a statement from the FBI.

The affidavit unsealed by investigators reveals some new details about the movements of the two individuals who were involved in the fatal shooting of Border Patrol agent David Maland.

Since Jan. 14, investigators had been conducting “periodic surveillance” on Youngblut and Felix Baukholt, a German national who was killed during the shooting, after they were reported as suspicious by someone who was staying at the same hotel as the pair, according to court records.

“An employee of a hotel in Lyndonville, Vermont contacted law enforcement after a male and a female had checked into the hotel to report concerns about them, including that they appeared to be dressed in all-black tactical style clothing with protective equipment, with the woman, later identified as Youngblut, carrying an apparent firearm in an exposed-carry holster,” according to charges unsealed in Vermont federal court on Friday.

Agents from the Vermont State Police and Homeland Security Investigations attempted to approach the pair for a voluntary interview, but they rebuffed law enforcement, “claiming that they were in the vicinity to look at purchasing property,” according to the court records.

Five days later, the pair were spotted in downtown Newport, Vermont, in “similar tactical dress” and carrying a handgun, which is permitted in Vermont, the affidavit said.

On Monday, the day of the fatal shooting, the two were being observed by law enforcement in the parking lot of a Walmart nearby, prosecutors said. Investigators observed Baukholt come out of the store with rolls of aluminum foil that he then used to wrap cellphones, they said.

The stop was initiated by Border Patrol agents under the pretext of an immigration violation, according to the court records.

Between 3 p.m. and 3:15 p.m. agents reported gunshots from the scene, according to the affidavit.

Agents described to their supervisor “that both Baukholt and Youngblut possessed firearms and that Youngblut drew and fired a handgun toward at least one of the uniformed Border Patrol agents without warning when outside the driver’s side of the Prius. Baukholt then attempted to draw a firearm. At least one Border Patrol agent fired at Youngblut and Baukholt with his service weapon,” according to the affidavit.

Upon searching the vehicle, agents found a cache of weapons and tactical gear, as well as the cellphones wrapped in foil.

The agents have not been interviewed according to the complaint, officials said.

“Agent Maland bravely served his country as a member of the U.S. Air Force,” said Special Agent in Charge Craig Tremaroli of the FBI Albany Field Office in a statement. “He continued that service when he answered the call to protect and serve as a law enforcement officer, making him a shining example of service over self. This arrest proves the FBI, together with our partners, will work diligently to ensure any individual who uses a firearm to assault such a public servant will be brought to justice.”

ABC News’ Alex Faul contributed to this report.