"But at the time it was only that -- a joke," he said.
The witness and his children watched casually as the mysterious object approached silently, until it struck them that it was an unusual kind of aircraft.
"Then the light started to dim out and we could see a dark form over it," according to the witness.
When his son realized the object was a "real UFO" he became agitated.
The mysterious square-shaped object cruised in the sky in the direction of their car, approaching within 300 meters.
It was dark colored and had rounded corners, the witness said.
"My son then started to literally freak out," he recalled. "That thing came about 300 meters toward us and it was a dark square with rounded corners thing, cruising the sky, no sound whatsoever."
He whipped out his phone and began filming (see video above). But he regretted the video failed to capture the sighting effectively.
"The video doesn't reflect even 10 percent of the clarity of the sighting," he said. "Unfortunately phone cameras are horrible, especially at dark. But it was so clear to us."
The witness said the UFO was about the size of a car, and it glided, smoothly and silently, with ease across the night sky. The dark color meant it was almost the same color as the night sky.
"It seemed to be the size of a car just cruising there, with almost the same color of the background," he wrote.
He recalled that his son was shaken by the encounter. His son had always been skeptical about UFOs, aliens and the paranormal, the witness explained.
"My son is still shaking," he wrote. "He never believed in anything paranormal."
MUFON said the organization's representatives in Illinois were investigating the sighting.
Although sightings or square- or rectangle-shaped UFO are relatively uncommon, compared with triangle, orb and cylinder- or cigar-shaped UFOs, there have been a few incidents in recent years that were remarkably similar to the latest reported sighting from Illinois.
Earlier on June 1, a private pilot from Sumerduck, a village in Fauquier County, Virginia, reported a 1993 sighting that involved a square- or rectangle-shaped UFO hovering 200 feet over a car dealership at the intersection of US 17 and Country Route 634.
He was driving towards the intersection with his wife, and the car was about 100 feet from the dealership, when they noticed the rectangle-shaped craft hovering silently.
He had thought it was a helicopter from Quantico Marine Base, but he realized he was mistaken when they came closer.
"Both my wife and I heard no sounds emanating from the craft," the witness reported. "[We had] the hairs on our bodies standing straight up during the incident."
"The rectangular light pattern was approximately 40 feet by 80 feet. The light pattern did not change during the whole incident," he added.
Earlier on March 30, 2016, a witness in Seattle reported watching a square-shaped UFO floating silently over Space Needle, while standing outside at about 8:00 p.m. on a terrace at the eighth-floor of the Hyatt House Seattle/Downtown hotel.
He first mistook the object for a fast-moving jet, but he soon noticed it was flying silently.
"Not a sound throughout sighting," the astonished witness told MUFON. "[It had] two white, blinking lights pulsating in unison at approximately 1 Hz, as if each wing, left and right, had a big, white light covering most of the wing surface. I could not discern forward fuselage or tail hence looked like it was a flying straight wing."
[Image via Shutterstock]