The investigation is looking into the deaths of a Marine and a Navy medic killed by a Hellfire missile fired from a Predator after they apparently were mistaken for insurgents in southern Afghanistan last week, two senior US defense officials said Tuesday. The officials spoke on condition of anonymity because an investigation is under way.

Unmanned aircraft have proved to be powerful weapons in Afghanistan and Iraq, and their use has expanded to new areas and operations each year of those conflicts. Some drones are used for surveillance, and some, such as the drone in this case, are armed and have been used to hunt and attack militants.

Officials said this is the first case they know of in which a drone may have been involved in a friendly fire incident in which US troops were killed. They are trying to determine how it happened.

Marine Staff Sgt. Jeremy Smith of Arlington, Texas, and Seaman Benjamin D. Rast of Niles, Michigan, were hit while moving toward other Marines who were under fire in Helmand province.

Military officials in Afghanistan would provide no details, saying only that it was a friendly fire incident.

“A formal investigation will determine the circumstances that led to the incident,” the International Security Assistance Force said in a statement last week.

Reports from the field indicate that the Marines who were under attack mistook Smith and Rast for militants heading their way and called in a strike from a US Air Force Predator, one official said.

Smith, 26, and Rast, 23, were with the 1st Battalion, 23rd Marine Regiment reserve unit in Houston.

Smith’s father, Jerry, said he did not want to place blame for what happened.

“Whoever that young man or woman was, they didn’t send that drone over there to kill my son or Doc Rast,” Smith’s father, Jerry, told the Fort Worth Star-Telegram newspaper.

“If it was a royal screwup, it was a royal screwup. Make corrections because I don’t want another family to have to go through this.” He said “trying to put a bigger burden on that person who fired the missile is not something I would do.” “I guarantee you if he was standing in front of me, he’d be asking for forgiveness, and I would give it to him,” Smith said.

The Marines have been in a fierce fight in the south, battling insurgents in major Taleban sanctuaries such as Sangin, a militant stronghold.

Currently, Air Force Predators and Reapers, the high-flying hunter-killer drones, are logging 48 of the 24-hour air patrols a day, moving toward a goal of 65 in 2013. The aircraft are prized for their intelligence-gathering proficiency and ability to pinpoint targets, reducing the risk to US pilots and other personnel.

Under pressure from Defense Secretary Robert Gates, the Air Force has dramatically increased the number of armed and unarmed drones in the war zones over the past three years.

Other military services have their own drones, ranging from the Army’s smaller shoulder-launched Ravens to the sophisticated, high-altitude Global Hawks, which are used for surveillance missions and do not carry weapons.

The military’s use of armed drones in Afghanistan has become a flash point for Afghan anger over civilian deaths in the nearly 10-year-old war.

Drones are an even more contentious weapon in Pakistan, where they are largely operated by the CIA to strike insurgents hiding along the border. Pakistan tacitly allows the drone strikes within limited areas but denies in public that it permits the Americans such leeway.