Just what the heck caused that giant rumble around Isanti yesterday?
Here's the mayor's post:
It got more than 120 comments, with people noting it shook windows in Bethel, rumbled houses and confused dogs.
People were wondering if it was a tree falling, a train, a crash, an explosion of some sort ...
The Isanti County Sheriff's Office jumped in with an answer shortly after, writing on the thread and offering a longer explanation on their own Facebook page.
An explosive device on ATK's testing grounds went off, according to the sheriff's office.
"There are NO emergency situations pertaining to this explosion that was heard," they wrote. "They do test these devices there [at ATK], this time it was an accidential [sic] discharge. No police or EMS were needed."
The grounds are in Anoka County, but just across the border from Isanti, west of St. Francis, the sheriff's office says.
FOX 9 also spoke with ATK about what exactly was being detonated.
So what is ATK?
It's an aerospace and defense technologies company, according to the website. Among the products they build: "tactical missiles, subsystems and defense electronics; and precision weapons, armament systems and ammunition."
The company has three Minnesota locations: Minnetonka, Plymouth and Elk River.