SACRAMENTO (CBS13) — A Cold War-era relic was responsible for that loud siren blaring across several Sacramento neighborhoods for more than a half-hour Monday night.

People had no idea where it was coming from. The late-night noise startled hundreds of people in Sacramento.

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“We opened the door and it was extremely loud,” said Chris Carstens, a Sacramento resident.

Many were rattled from bed by the alarming sound. People in Sacramento’s Tahoe Park neighborhood say the siren was coming from the nearby SMUD property. The utility even tweeted out that it appeared to be a building alarm.

But that’s not what it turned out to be.

“It was frustrating no one had an explanation for it,” Sacramento resident Jennifer Fearing said.

So just what was making the loud noise? A Cold War-era air-raid siren was the culprit.

It was one of dozens of civil defense sirens installed across the city back in the 1940s to warn people of an imminent emergency like a dam break or nuclear war.

And it can be heard from more than a mile away.

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“My husband and I joked about getting under a table,” Fearing said.

In a real emergency, these sirens would have been activated by 9-1-1 dispatchers. But the system was decommissioned years ago and a dispatcher told CBS13 they don’t even have the equipment to turn them on anymore.

Now, emergency alerts are sent out by tv, radio, and cellphones.

Still, more than a dozen of these yellow towers remain across Sacramento with even some longtime residents unaware they exist.

“I’ve lived in Sacramento my entire life and I’ve never been told we have an air raid system,” Carstens said.

“It seems like if that’s not a system we’re using anymore, someone ought to permanently cut the power to it,” Fearing said.

SMUD says it eventually did cut the power to the air raid alarm.

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The city is still trying to determine exactly what caused the siren to activate.