Suspect in killing of Vermont Border Patrol agent watched for days, had cell phones wrapped in foil

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 about the suspects’ movements were previously revealed in the arrest affidavit.

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

The affidavit released by investigators reveals some new details about the movements of the two people 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 in the shooting, after they were reported as suspicious by someone staying at the same hotel as the couple, according to court records.

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

Agents from the Vermont State Police and Homeland Security Investigations tried to approach the couple for a voluntary interview, but they turned law enforcement away, “claiming they were in the area to look at buying property,” according to court records.

Five days later, the couple was seen in downtown Newport, Vermont, wearing “similar tactical dress” and carrying a handgun, which is allowed in Vermont, the affidavit said.

On Monday, the day of the fatal shooting, the two were observed by police in the parking lot of a nearby Walmart, prosecutors said. Investigators observed Baukholt exit the store with rolls of aluminum foil, which he then used to wrap cell phones, they said.

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

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

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

After searching the vehicle, agents found a cache of weapons and tactical equipment, as well as cell phones 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 United States Air Force,” Special Agent Craig Tremaroli of the FBI Albany Field Office said in a statement. “He continued that service when he answered the call to protect and serve as law enforcement, making him a shining example of service above self. This arrest proves that the FBI, along with our partners, will work diligently to secure every person, who uses a firearm to attack such public servant will be brought to justice.”

ABC News’ Alex Faul contributed to this report.