PHOENIX -- Phoenix police and media outlets were flooded with calls Monday night after people spotted four or five red colored lights hovering in the sky over north Phoenix.

Callers said the lights appeared at one point in a straight line, and also formed a square and then a triangle. They were visible for about 15 minutes around 8 p.m. before heading to the east and disappearing.

Officials at Deer Valley Airport in north Phoenix and air traffic controllers at Sky Harbor International east of downtown Phoenix were among those who saw the lights. But they couldn't explain them, and the Air Force said the lights weren't part of any Air Force activities.

A Luke Air Force Base official said the base wasn't flying any aircraft in the sky Monday night.

Authorities with the Marine Corps Air Station in Yuma and Arizona National Guard also said their agencies had not been conducting any air operations.

A spokesman for the North American Aerospace Defense Command, which monitors information on missiles or other manmade objects re-entering the atmosphere, said the agency received no reports of activity in the region.

Ian Gregor, a spokesman for the Federal Aviation Administration, said they do not know the cause.

"There was nothing unusual on our radar scopes. There were no unidentified aircraft on our radar scopes. This is a non-issue for us," Gregor said. "There's nothing to investigate."

The incident was reminiscent of one that occurred on the night of March 13, 1997. That night thousands of people reported seeing a mile-wide, v-shaped formation of lights over the Phoenix area.

Dozens of listeners called News/Talk 92-3 KTAR just after 8 p.m. reporting they were watching the four mystery lights.

"From my position, it looked like they were just hanging, not moving at all," said one man, who called 92-3's "Gaydos After Dark." He said he "absolutely" saw something.

A woman caller said, "It looked like four red tower lights, but it was pretty high up in the air. I called my husband and he said, 'Get home, what's wrong with you?'"

A man in north Phoenix told CBS5 News: "They were about 3,000 feet high, approximately. They looked as though they were kind of hovering or floating from west to east, very slowly. They were up there for 15 or 20 minutes."