Loud siren noise blared across East Sacramento, Tahoe Park on Monday night. What was it?

A loud, droning sound startled residents in East Sacramento, Tahoe Park and other nearby neighborhoods late Monday night.

What was it?

Sacramento Municipal Utility District officials wrote in a social media post around 11 p.m. that it was investigating to see if it was a building alarm at its 59th Street maintenance campus.

Others began speculating on social media that the sound was a malfunctioning air raid siren.

The city of Sacramento has about two dozen civil defense sirens, according to a 2008 city evacuation plan. They were installed during World War II as an air raid alert system, according to the plan.

One of the sirens is located at 1708 59th St., an address for one of SMUD’s business office that fronts a large maintainance yard, according to the 2008 document.

“I can confirm that the siren is not owned by SMUD, however it is on our 59th Street property,” SMUD spokeswoman Lindsay VanLaningham wrote in response to an emailed question about the air raid siren. “We were able to cut the power to it (Monday) night.”

It remains unclear what triggered the siren Monday. The civil defense sirens are now essentially dormant, no longer tested on a monthly basis as they used to be more than a decade ago.

Most city and county emergency alert messages are now transmitted instead through radio, television, phone and SMS text message alerts, which effectively rendered the sirens obsolete.

No emergency alert notifications, through the Sacramento Region Emergency Notification System or other channels, appear to have gone out Monday night.