What can only be described as a "loud boom" has baffled police in Lochbuie and Brighton - and caused hundreds of residents to wonder, "What was that?"
Well, the Chief of Lochbuie Police, Tracey McCoy, says they still don't know.
Around 9 p.m. Monday, Lochbuie Police say their officers were notified of a 'suspicious sound' that was described as a 'loud boom.'
Police report the 'boom' shook multiple houses and appeared to originate on the northwest side of Lochbuie.
However, the police can't seem to figure out what caused the mysterious noise.
Chief McCoy said Lochbuie Police have reached out to the FAA and various military contacts, but neither organization has any information about the reported boom.
Brighton Police also received similar reports as far as south of Bridge Street.
On Tuesday morning, a spokesperson for Brighton PD said officers did investigate a few calls, but did not discover a source of the noise.
Similarly, a public information officer for Brighton Fire Department said on Tuesday morning that their crews were dispatched to a home Monday night in the northwest part of Lochbuie, at 197 Jewell St.
The caller had reported a 'loud explosion,' but when firefighters arrived, there was no smoke or fire anywhere nearby, just lots of neighbors in the street, wondering what had happened.
Brighton Fire cleared the scene without finding a source of the noise around 11 p.m.
A USGS geophysicist told 9NEWS that a seismograph station at the Rocky Mountain Arsenal recorded a "signal" at the same time as the loud boom.
But, this may or may not be related, because it wasn't similar to the burst you'd see during an explosion, but rather a constant signal. It probably wouldn't have been recorded at all had there not been a station nearby, the USGS said.
As soon as Lochbuie Police posted on Facebook about the mystery, hundreds of residents began commenting with what they experienced.
Dale Kutin wrote, "Brighton Crossings here and we thought a car hit our house. It was like a shockwave from a BIG explosion but we didn’t hear anything, just felt it."
From Debbie Stewart, "In Brighton east farms, we thought something fell off of an airplane and hit house. Went outside and looked around."
Everyone seemed to be curious - and relieved that others had reported the noise, too.
Leslie Denise Dimas said, "My whole house shook, floor and things rattled, I'm over by county road 8, was baffled by the noise. Thank you for posting this, greatly appreciate knowing I wasn't the only one!"
Still, as of Tuesday morning, there was no indication police knew what the sound was.
Chief McCoy said there was no reports of injury nor property damage in Lochbuie Tuesday night. Since the calls spanned "quite a distance," the chief says they are working with nearby jurisdictions to try to determine the boom's cause.
Ultimately, he said, they just may not be able to figure it out. However, McCoy said the department would continue to research as best as they can.
"We'll keep our eyes and ears open," he said by phone Tuesday.
In his six years with the department, Chief McCoy says he's never had a call like this before.
Lochbuie is a town of fewer than 5,000 people located Northeast of Brighton in Weld and Adams Counties.
There is no criminal investigation into the sound, Chief McCoy said Tuesday.