A line of flashing lights drew the attention of plenty of people on Saturday night.
Some people in Billings and other areas of southern Montana also saw the lights, including Three Forks and Butte.
Don Larson told MTN News affiliate KTVQ that the planes were over the Billings area around 8:15 p.m. on Saturday.
According to CBS affiliate WLTX, a spokesperson for Joint Base Charleston said on Sunday morning that a group of 17 C-17 cargo jets with the 437th Airlift Wing took off from their base around 7 p.m. Saturday.
The planes flew cross-country to Nevada for a training exercise involving HALO drops--high altitude military parachuting. The exercise was done in coordination with multiple air military installations and groups, including the 82nd Airborne. As the group moved westward, more C-17s and other aircraft joined the effort.
Aviation expert Steve Cowell told CBS News that the planes were flying in a formation that is commonly seen in military exercises.
"That was the echelon formation. In other words, they were all lined up in a row," Cowell said. "It is not uncommon to have these types of planes in formation, especially at night. Often times, you will see them doing this, especially when they are doing training flights, or standard cross country procedure."
Some described the line of lights in the sky as "creepy," claiming they couldn't hear any sounds coming from more than a dozen aircraft. Cowell said the planes, being military cargo aircraft, are designed to operate at quiet tones.
"(C-17s) have been modified to be just a little quieter. They are also flying a little slower. At that kind of power, you are not going to hear a lot of noise," Cowell said.
While it may have been fun to speculate the possibility of extraterrestrial life making its way to Earth, Cowell said the lights in the sky over Denver were not extremely rare.
"Don't assume it is a UFO. Not until the bulletin comes over the news," Cowell said.