Fireworks and electrical transformers account for only half of the booms resonating again across south Minneapolis.
The enduring mystery of the south Minneapolis explosions rattling both windows and neighbors' nerves has once again reared its head.
A new spate of nighttime blasts, roughly 100, have been going off since summer, something that has been occurring intermittently for nearly three years.
The last time that police investigated the spate of explosions, in 2006 and 2007, they were finally able to determine the source: fireworks, most likely set off by teenagers.
This time, though, only about half can be explained, said Lt. Dean Christiansen, who's coordinating the investigation in the Third Precinct.
Fireworks and exploding electrical transformers account for the explained half, "but for the rest, we just don't know," he said. "We can't explain it."
The most troubling, if far-fetched, theory -- that anarchists were in the Mississippi River gorge, practicing their explosive skills in preparation for the Republican National Convention -- didn't pan out.
"It was a real homeland security concern so we were down there in the river with the St. Paul cops, but that wasn't it," Christiansen said.
Undercover cops have been working the neighborhoods where the blasts have been reported but have enjoyed only mixed success.
For example, on Monday shortly after midnight, three explosions were reported and were quickly determined to be fireworks. Two more reported several hours later remain head scratchers.
"There's one theory that competing groups of some kind are trying to see who can come up with the loudest explosions down by the river," Christiansen said.
Another theory that didn't hold was the possibility that some unknown kind of chemical reaction was occurring in the city's water treatment system.
Although most appear to be occurring near the river, sound echo patterns have sent the noise across a wide swath of the city along the river from roughly E. Lake Street to Ford Parkway. And they've been heard by residents dozens of blocks to the west.
Just as in 2007, news of the explosions has spread like electronic wildfire among residents, who have lit up neighborhood e-mail lists with their accounts of the noise. Last time, theories ran from pipe bombs to sonic booms to exploding gas lines.
This time, the theories have run more along the lines of propane cannons and violent freight train car coupling.
That was offered by Standish-Ericsson neighborhood resident Geoff Garton, who added, "sounds can play some amazing tricks, ricocheting around the city."
Police are continuing their investigation, Christiansen said.
Bob von Sternberg 612-673-7184