The father of one of the two pilots on board the plane that collided with the military helicopter told Fox 5 in Atlanta that he is grieving the death of his 28-year-old son, Sam Lilley.

“This is undoubtedly the worst day of my life,” Timothy Lilley said.

Sam Lilley, of Savannah, Ga., was engaged and planned to get married in the fall, his father said.

Timothy Lilley described himself as a longtime former Army helicopter pilot who is familiar with the air traffic in the Washington, D.C., area.

“I was a helicopter pilot in the Army for 20 years. In the ’90s, I used to fly in and out of the Pentagon regularly, and I can tell you if you are flying on the route over the Potomac and wearing night vision goggles, it’s going to be very hard to see that plane. If you’re not wearing the goggles, then you might have a chance,” he said.

Investigators have yet to determine what caused the deadly collision. U.S. officials said that the helicopter was on a routine nighttime training mission and that the crew was equipped with night vision goggles, but it’s unclear whether they were using them at the time of the incident.

“From what I can see, those guys turned right into the jet,” Timothy Lilley said. “I think the PSA jet was doing everything right. The Army pilot made a grave error. It hurts me because those are my brothers, and now my son is dead.”