The Federal Bureau of Investigation is looking into a spate of mysterious robocalls urging people to stay home on Election Day, a Department of Homeland Security official said. The FBI did not immediately return messages seeking comment. U.S. state and local officials have been raising the alarm over at least two separate automated call campaigns, whose origins and exact purpose are unclear. Experts who spoke to Reuters say they are mystified by one of the campaigns, which tells people to remain home but does not explicitly mention voting. “There’s a little bit of confusion about this one across the industry,” said Giulia Porter, vice president at RoboKiller, a company that fights telemarketers and robocalls and has been tracking the campaign. Audio of the calls, which RoboKiller shared, features a synthetic female voice saying: “Hello. This is just a test call. Time to stay home. Stay safe and stay home.” Porter said the call had been placed millions of times in the past 11 months or so but had just today shot up to No. 5 or No. 6 in the list of top spam calls. “This robocall is being sent at a very high volume,” she said. She said her company was still in the process of compiling figures on the campaign’s intensity on Tuesday but estimated that “thousands or tens of thousands” of people had received it.