Stillwater, Minn. (AP) — The Washington County sheriff’s office is investigating after some warning sirens failed to sound during Sunday’s severe weather.

After the National Weather Service issued a tornado warning Sunday, the sheriff’s office activated the outdoor warning sirens for central and northern Washington County. But first responders reported that sirens did not go off in some cities. A backup system also failed.

When a second tornado warning was issued, the 911 communications center was still trying to troubleshoot the problem. Dispatchers were able to use a backup system to send a signal, but by that time it was too late.

Washington County checked with its vendor and discovered there was an equipment failure at the county’s uninterruptible power supply. The county plans to test the sirens at 1 p.m. Wednesday.

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