LOS ANGELES (AP) — People in San Luis Obispo County received a series of unsettling, erroneous emergency alerts Friday as repairs were being made to a nuclear power plant’s siren system, including a vague cellphone message that told them to “prepare for action.”

The chain of mistaken alerts began arousing confusion and fear when a siren that’s part of the Diablo Canyon Power Plant’s warning system began wailing Friday afternoon for no apparent reason, county emergency services manager Ron Alsop said.

Earlier in the day, crews had upgraded the siren as part of Pacific Gas & Electric Co.’s summer-long revamp of the emergency system.

To indicate there was no emergency, county officials issued an alert.

“Unbeknownst to us, with a new emergency alert system, it also triggered the new wireless alert cellphone system,” Alsop said.

Across the county, people’s cellphones buzzed with a special tone and a message that said there was a “civil emergency in this area” and people should “prepare for action.”

The warning halted wine tasting at the Saucelito Canyon Vineyard & Winery in San Luis Obispo, manager Katherine Taylor said.

“I had 15 people here, looking at their phones and asking what to do,” she said.

Nisse Noble, 27, was at her online apparel company’s office when she received the message. The vague warning was “unsettling,” leading her to think there was a mass shooting, a nuclear accident or a criminal at large in the area, she said.

“We didn’t know where we should turn or what we should prepare for,” Noble said.

When county officials realized the gaffe, they issued a final message stating that there was no cause for alarm.

“We’re just glad we learned this before we had an actual emergency,” Alsop said.

The 131 sirens across the county are used not only for an emergency at the nuclear plant, but also for other emergencies such as an evacuation due to a tsunami or dam failure, PG&E said. The utility said it was investigating what caused the siren to sound.