WASHINGTON — A Pentagon investigation into the death of an American soldier in Afghanistan is looking at whether an Army brigade that is at the center of the Trump administration’s new war strategy had followed proper security procedures designed to protect United States troops from insider attacks by Afghan forces they train.

Cpl. Joseph Maciel, who was killed on July 7, was part of a group of soldiers assigned to protect American military advisers with the First Security Forces Assistance Brigade. He was with a team of roughly 25 trainers and soldiers that was attacked in Tarinkot, a town in the Taliban heartland, by what a local councilman described as an Afghan soldier who fired on the Americans.

The brigade’s roughly 1,000 soldiers are among the first conventional American forces since 2014 that are being sent into active fighting zones. They are spread across the country in small teams to help Afghan troops with training, intelligence and logistics.

Senior Pentagon officials, including Gen. Mark A. Milley, the Army chief of staff, and commanders at the American-led coalition’s headquarters in Kabul have declined to discuss details of the deadly shooting, which also wounded two American soldiers and an Afghan interpreter.